Using SSH keys inside docker container

Question

I have an app that executes various fun stuff with Git (like running git clone & git push) and I'm trying to docker-ize it.

I'm running into an issue though where I need to be able to add an SSH key to the container for the container 'user' to use.

I tried copying it into /root/.ssh/, changing $HOME, creating a git ssh wrapper, and still no luck.

Here is the Dockerfile for reference:

#DOCKER-VERSION 0.3.4                                                           

from  ubuntu:12.04                                                              

RUN  apt-get update                                                             
RUN  apt-get install python-software-properties python g++ make git-core openssh-server -y
RUN  add-apt-repository ppa:chris-lea/node.js                                   
RUN  echo "deb http://archive.ubuntu.com/ubuntu precise universe" >> /etc/apt/sources.list
RUN  apt-get update                                                             
RUN  apt-get install nodejs -y                                                  

ADD . /src                                                                       
ADD ../../home/ubuntu/.ssh/id_rsa /root/.ssh/id_rsa                             
RUN   cd /src; npm install                                                      

EXPOSE  808:808                                                                 

CMD   [ "node", "/src/app.js"]

app.js runs the git commands like git pull

Answer 1

It's a harder problem if you need to use SSH at build time. For example if you're using git clone, or in my case pip and npm to download from a private repository.

The solution I found is to add your keys using the --build-arg flag. Then you can use the new experimental --squash command (added 1.13) to merge the layers so that the keys are no longer available after removal. Here's my solution:

Build command

$ docker build -t example --build-arg ssh_prv_key="$(cat ~/.ssh/id_rsa)" --build-arg ssh_pub_key="$(cat ~/.ssh/id_rsa.pub)" --squash .

Dockerfile

FROM python:3.6-slim

ARG ssh_prv_key
ARG ssh_pub_key

RUN apt-get update && \
    apt-get install -y \
        git \
        openssh-server \
        libmysqlclient-dev

# Authorize SSH Host
RUN mkdir -p /root/.ssh && \
    chmod 0700 /root/.ssh && \
    ssh-keyscan github.com > /root/.ssh/known_hosts

# Add the keys and set permissions
RUN echo "$ssh_prv_key" > /root/.ssh/id_rsa && \
    echo "$ssh_pub_key" > /root/.ssh/id_rsa.pub && \
    chmod 600 /root/.ssh/id_rsa && \
    chmod 600 /root/.ssh/id_rsa.pub

# Avoid cache purge by adding requirements first
ADD ./requirements.txt /app/requirements.txt

WORKDIR /app/

RUN pip install -r requirements.txt

# Remove SSH keys
RUN rm -rf /root/.ssh/

# Add the rest of the files
ADD . .

CMD python manage.py runserver

---

Update: If you're using Docker 1.13 and have experimental features on you can append --squash to the build command which will merge the layers, removing the SSH keys and hiding them from docker history.

Answer 2

Turns out when using Ubuntu, the ssh_config isn't correct. You need to add

RUN  echo "    IdentityFile ~/.ssh/id_rsa" >> /etc/ssh/ssh_config

to your Dockerfile in order to get it to recognize your ssh key.

Answer 3

Note: only use this approach for images that are private and will always be!

The ssh key remains stored within the image, even if you remove the key in a layer command after adding it (see comments in this post).

In my case this is ok, so this is what I am using:

# Setup for ssh onto github
RUN mkdir -p /root/.ssh
ADD id_rsa /root/.ssh/id_rsa
RUN chmod 700 /root/.ssh/id_rsa
RUN echo "Host github.com\n\tStrictHostKeyChecking no\n" >> /root/.ssh/config

Answer 4

If you are using docker compose an easy choice is to forward SSH agent like that:

something:
    container_name: something
    volumes:
        - $SSH_AUTH_SOCK:/ssh-agent # Forward local machine SSH key to docker
    environment:
        SSH_AUTH_SOCK: /ssh-agent

Answer 5

Expanding Peter Grainger's answer I was able to use multi-stage build available since Docker 17.05. Official page states:

With multi-stage builds, you use multiple FROM statements in your Dockerfile. Each FROM instruction can use a different base, and each of them begins a new stage of the build. You can selectively copy artifacts from one stage to another, leaving behind everything you don’t want in the final image.

Keeping this in mind here is my example of Dockerfile including three build stages. It's meant to create a production image of client web application.

# Stage 1: get sources from npm and git over ssh
FROM node:carbon AS sources
ARG SSH_KEY
ARG SSH_KEY_PASSPHRASE
RUN mkdir -p /root/.ssh && \
    chmod 0700 /root/.ssh && \
    ssh-keyscan bitbucket.org > /root/.ssh/known_hosts && \
    echo "${SSH_KEY}" > /root/.ssh/id_rsa && \
    chmod 600 /root/.ssh/id_rsa
WORKDIR /app/
COPY package*.json yarn.lock /app/
RUN eval `ssh-agent -s` && \
    printf "${SSH_KEY_PASSPHRASE}\n" | ssh-add $HOME/.ssh/id_rsa && \
    yarn --pure-lockfile --mutex file --network-concurrency 1 && \
    rm -rf /root/.ssh/

# Stage 2: build minified production code
FROM node:carbon AS production
WORKDIR /app/
COPY --from=sources /app/ /app/
COPY . /app/
RUN yarn build:prod

# Stage 3: include only built production files and host them with Node Express server
FROM node:carbon
WORKDIR /app/
RUN yarn add express
COPY --from=production /app/dist/ /app/dist/
COPY server.js /app/
EXPOSE 33330
CMD ["node", "server.js"]

.dockerignore repeats contents of .gitignore file (it prevents node_modules and resulting dist directories of the project from being copied):

.idea
dist
node_modules
*.log

Command example to build an image:

$ docker build -t ezze/geoport:0.6.0 \
  --build-arg SSH_KEY="$(cat ~/.ssh/id_rsa)" \
  --build-arg SSH_KEY_PASSPHRASE="my_super_secret" \
  ./

If your private SSH key doesn't have a passphrase just specify empty SSH_KEY_PASSPHRASE argument.

This is how it works:

1). On the first stage only package.json, yarn.lock files and private SSH key are copied to the first intermediate image named sources. In order to avoid further SSH key passphrase prompts it is automatically added to ssh-agent. Finally yarn command installs all required dependencies from NPM and clones private git repositories from Bitbucket over SSH.

2). The second stage builds and minifies source code of web application and places it in dist directory of the next intermediate image named production. Note that source code of installed node_modules is copied from the image named sources produced on the first stage by this line:

COPY --from=sources /app/ /app/

Probably it also could be the following line:

COPY --from=sources /app/node_modules/ /app/node_modules/

We have only node_modules directory from the first intermediate image here, no SSH_KEY and SSH_KEY_PASSPHRASE arguments anymore. All the rest required for build is copied from our project directory.

3). On the third stage we reduce a size of the final image that will be tagged as ezze/geoport:0.6.0 by including only dist directory from the second intermediate image named production and installing Node Express for starting a web server.

Listing images gives an output like this:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
ezze/geoport        0.6.0               8e8809c4e996        3 hours ago         717MB
<none>              <none>              1f6518644324        3 hours ago         1.1GB
<none>              <none>              fa00f1182917        4 hours ago         1.63GB
node                carbon              b87c2ad8344d        4 weeks ago         676MB

where non-tagged images correpsond to the first and the second intermediate build stages.

If you run

$ docker history ezze/geoport:0.6.0 --no-trunc

you will not see any mentions of SSH_KEY and SSH_KEY_PASSPHRASE in the final image.

Answer 6

In order to inject you ssh key, within a container, you have multiple solutions:

1. Using a Dockerfile with the ADD instruction, you can inject it during your build process

2. Simply doing something like cat id_rsa | docker run -i <image> sh -c 'cat > /root/.ssh/id_rsa'

3. Using the docker cp command which allows you to inject files while a container is running.

Answer 7

One cross-platform solution is to use a bind mount to share the host's .ssh folder to the container:

docker run -v /home/<host user>/.ssh:/home/<docker user>/.ssh <image>

Similar to agent forwarding this approach will make the public keys accessible to the container. An additional upside is that it works with a non-root user too and will get you connected to GitHub. One caveat to consider, however, is that all contents (including private keys) from the .ssh folder will be shared so this approach is only desirable for development and only for trusted container images.

Answer 8

Docker containers should be seen as 'services' of their own. To separate concerns you should separate functionalities:

1) Data should be in a data container: use a linked volume to clone the repo into. That data container can then be linked to the service needing it.

2) Use a container to run the git cloning task, (i.e it's only job is cloning) linking the data container to it when you run it.

3) Same for the ssh-key: put it is a volume (as suggested above) and link it to the git clone service when you need it

That way, both the cloning task and the key are ephemeral and only active when needed.

Now if your app itself is a git interface, you might want to consider github or bitbucket REST APIs directly to do your work: that's what they were designed for.

Answer 9

Forward the ssh authentication socket to the container:

docker run --rm -ti \
        -v $SSH_AUTH_SOCK:/tmp/ssh_auth.sock \
        -e SSH_AUTH_SOCK=/tmp/ssh_auth.sock \
        -w /src \
        my_image

Your script will be able to perform a git clone.

Extra: If you want cloned files to belong to a specific user you need to use chown since using other user than root inside the container will make git fail.

You can do this publishing to the container's environment some additional variables:

docker run ...
        -e OWNER_USER=$(id -u) \
        -e OWNER_GROUP=$(id -g) \
        ...

After you clone you must execute chown $OWNER_USER:$OWNER_GROUP -R <source_folder> to set the proper ownership before you leave the container so the files are accessible by a non-root user outside the container.

Answer 10

Starting from docker API 1.39+ (Check API version with docker version) docker build allows the --ssh option with either an agent socket or keys to allow the Docker Engine to forward SSH agent connections.

Build Command

export DOCKER_BUILDKIT=1
docker build --ssh default=~/.ssh/id_rsa .

Dockerfile

# syntax=docker/dockerfile:experimental
FROM python:3.7

# Install ssh client (if required)
RUN apt-get update -qq
RUN apt-get install openssh-client -y

# Download public key for github.com
RUN --mount=type=ssh mkdir -p -m 0600 ~/.ssh && ssh-keyscan github.com >> ~/.ssh/known_hosts

# Clone private repository
RUN --mount=type=ssh git clone [email protected]:myorg/myproject.git myproject

More Info:

• https://docs.docker.com/develop/develop-images/build_enhancements/#using-ssh-to-access-private-data-in-builds
• https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/experimental.md#run---mounttypessh

Answer 11

This line is a problem:

ADD ../../home/ubuntu/.ssh/id_rsa /root/.ssh/id_rsa

When specifying the files you want to copy into the image you can only use relative paths - relative to the directory where your Dockerfile is. So you should instead use:

ADD id_rsa /root/.ssh/id_rsa

And put the id_rsa file into the same directory where your Dockerfile is.

Check this out for more details: http://docs.docker.io/reference/builder/#add

Answer 12

We had similar problem when doing npm install in docker build time.

Inspired from solution from Daniel van Flymen and combining it with git url rewrite, we found a bit simpler method for authenticating npm install from private github repos - we used oauth2 tokens instead of the keys.

In our case, the npm dependencies were specified as "git+https://github.com/..."

For authentication in container, the urls need to be rewritten to either be suitable for ssh authentication (ssh://[email protected]/) or token authentication (https://${GITHUB_TOKEN}@github.com/)

Build command:

docker build -t sometag --build-arg GITHUB_TOKEN=$GITHUB_TOKEN . 

Unfortunately, I'm on docker 1.9, so --squash option is not there yet, eventually it needs to be added

Dockerfile:

FROM node:5.10.0

ARG GITHUB_TOKEN

#Install dependencies
COPY package.json ./

# add rewrite rule to authenticate github user
RUN git config --global url."https://${GITHUB_TOKEN}@github.com/".insteadOf "https://github.com/"

RUN npm install

# remove the secret token from the git config file, remember to use --squash option for docker build, when it becomes available in docker 1.13
RUN git config --global --unset url."https://${GITHUB_TOKEN}@github.com/".insteadOf

# Expose the ports that the app uses
EXPOSE 8000

#Copy server and client code
COPY server /server 
COPY clients /clients

Answer 13

I ran into the same problem today and little bit modified version with previous posts I found this approach more useful to me

docker run -it -v ~/.ssh/id_rsa:/root/.my-key:ro image /bin/bash

(Note that readonly flag so container will not mess my ssh key in any case.)

Inside container I can now run:

ssh-agent bash -c "ssh-add ~/.my-key; git clone <gitrepourl> <target>"

So I don't get that Bad owner or permissions on /root/.ssh/.. error which was noted by @kross

Answer 14

You can use multi stage build to build containers This is the approach you can take :-

Stage 1 building an image with ssh

FROM ubuntu as sshImage
LABEL stage=sshImage
ARG SSH_PRIVATE_KEY
WORKDIR /root/temp

RUN apt-get update && \
    apt-get install -y git npm 

RUN mkdir /root/.ssh/ &&\
    echo "${SSH_PRIVATE_KEY}" > /root/.ssh/id_rsa &&\
    chmod 600 /root/.ssh/id_rsa &&\
    touch /root/.ssh/known_hosts &&\
    ssh-keyscan github.com >> /root/.ssh/known_hosts

COPY package*.json ./

RUN npm install

RUN cp -R node_modules prod_node_modules

Stage 2: build your container

FROM node:10-alpine

RUN mkdir -p /usr/app

WORKDIR /usr/app

COPY ./ ./

COPY --from=sshImage /root/temp/prod_node_modules ./node_modules

EXPOSE 3006

CMD ["npm", "run", "dev"] 

add env attribute in your compose file:

   environment:
      - SSH_PRIVATE_KEY=${SSH_PRIVATE_KEY}

then pass args from build script like this:

docker-compose build --build-arg SSH_PRIVATE_KEY="$(cat ~/.ssh/id_rsa)"

And remove the intermediate container it for security. This Will help you cheers.

Answer 15

This issue is really an annoying one. Since you can't add/copy any file outside the dockerfile context, which means it's impossible to just link ~/.ssh/id_rsa into image's /root/.ssh/id_rsa, and when you definitely need a key to do some sshed thing like git clone from a private repo link..., during the building of your docker image.

Anyways, I found a solution to workaround, not so persuading but did work for me.

1. in your dockerfile:

   • add this file as /root/.ssh/id_rsa
   • do what you want, such as git clone, composer...
   • rm /root/.ssh/id_rsa at the end

2. a script to do in one shoot:

   • cp your key to the folder holding dockerfile
   • docker build
   • rm the copied key

3. anytime you have to run a container from this image with some ssh requirements, just add -v for the run command, like:

   docker run -v ~/.ssh/id_rsa:/root/.ssh/id_rsa --name container image command

This solution results in no private key in both you project source and the built docker image, so no security issue to worry about anymore.

Answer 16

As eczajk already commented in Daniel van Flymen's answer it does not seem to be safe to remove the keys and use --squash, as they still will be visible in the history (docker history --no-trunc).

Instead with Docker 18.09, you can now use the "build secrets" feature. In my case I cloned a private git repo using my hosts SSH key with the following in my Dockerfile:

# syntax=docker/dockerfile:experimental

[...]

RUN --mount=type=ssh git clone [...]

[...]

To be able to use this, you need to enable the new BuildKit backend prior to running docker build:

export DOCKER_BUILDKIT=1

And you need to add the --ssh default parameter to docker build.

More info about this here: https://medium.com/@tonistiigi/build-secrets-and-ssh-forwarding-in-docker-18-09-ae8161d066

Answer 17

'you can selectively let remote servers access your local ssh-agent as if it was running on the server'

https://developer.github.com/guides/using-ssh-agent-forwarding/

Answer 18

A concise overview of the challenges of SSH inside Docker containers is detailed here. For connecting to trusted remotes from within a container without leaking secrets there are a few ways:

• SSH agent forwarding (Linux-only, not straight-forward)
• Inbuilt SSH with BuildKit (Experimental, not yet supported by Compose)
• Using a bind mount to expose ~/.ssh to container. (Development only, potentially insecure)
• Docker Secrets (Cross-platform, adds complexity)

Beyond these there's also the possibility of using a key-store running in a separate docker container accessible at runtime when using Compose. The drawback here is additional complexity due to the machinery required to create and manage a keystore such as Vault by HashiCorp.

For SSH key use in a stand-alone Docker container see the methods linked above and consider the drawbacks of each depending on your specific needs. If, however, you're running inside Compose and want to share a key to an app at runtime (reflecting practicalities of the OP) try this:

• Create a docker-compose.env file and add it to your .gitignore file.
• Update your docker-compose.yml and add env_file for service requiring the key.
• Access public key from environment at application runtime, e.g. process.node.DEPLOYER_RSA_PUBKEY in the case of a Node.js application.

The above approach is ideal for development and testing and, while it could satisfy production requirements, in production you're better off using one of the other methods identified above.

Additional resources:

• Docker Docs: Use bind mounts
• Docker Docs: Manage sensitive data with Docker secrets
• Stack Overflow: Using SSH keys inside docker container
• Stack Overflow: Using ssh-agent with docker on macOS

Answer 19

You can also link your .ssh directory between the host and the container, I don't know if this method has any security implications but it may be the easiest method. Something like this should work:

$ sudo docker run -it -v /root/.ssh:/root/.ssh someimage bash

Remember that docker runs with sudo (unless you don't), if this is the case you'll be using the root ssh keys.

Answer 20

If you don't care about the security of your SSH keys, there are many good answers here. If you do, the best answer I found was from a link in a comment above to this GitHub comment by diegocsandrim. So that others are more likely to see it, and just in case that repo ever goes away, here is an edited version of that answer:

Most solutions here end up leaving the private key in the image. This is bad, as anyone with access to the image has access to your private key. Since we don't know enough about the behavior of squash, this may still be the case even if you delete the key and squash that layer.

We generate a pre-sign URL to access the key with aws s3 cli, and limit the access for about 5 minutes, we save this pre-sign URL into a file in repo directory, then in dockerfile we add it to the image.

In dockerfile we have a RUN command that do all these steps: use the pre-sing URL to get the ssh key, run npm install, and remove the ssh key.

By doing this in one single command the ssh key would not be stored in any layer, but the pre-sign URL will be stored, and this is not a problem because the URL will not be valid after 5 minutes.

The build script looks like:

# build.sh
aws s3 presign s3://my_bucket/my_key --expires-in 300 > ./pre_sign_url
docker build -t my-service .

Dockerfile looks like this:

FROM node

COPY . .

RUN eval "$(ssh-agent -s)" && \
    wget -i ./pre_sign_url -q -O - > ./my_key && \
    chmod 700 ./my_key && \
    ssh-add ./my_key && \
    ssh -o StrictHostKeyChecking=no [email protected] || true && \
    npm install --production && \
    rm ./my_key && \
    rm -rf ~/.ssh/*

ENTRYPOINT ["npm", "run"]

CMD ["start"]

Answer 21

A simple and secure way to achieve this without saving your key in a Docker image layer, or going through ssh_agent gymnastics is:

1. As one of the steps in your Dockerfile, create a .ssh directory by adding:

   RUN mkdir -p /root/.ssh

2. Below that indicate that you would like to mount the ssh directory as a volume:

   VOLUME [ "/root/.ssh" ]

3. Ensure that your container's ssh_config knows where to find the public keys by adding this line:

   RUN echo " IdentityFile /root/.ssh/id_rsa" >> /etc/ssh/ssh_config

4. Expose you local user's .ssh directory to the container at runtime:

   docker run -v ~/.ssh:/root/.ssh -it image_name

   Or in your dockerCompose.yml add this under the service's volume key:

   - "~/.ssh:/root/.ssh"

Your final Dockerfile should contain something like:

FROM node:6.9.1

RUN mkdir -p /root/.ssh
RUN  echo "    IdentityFile /root/.ssh/id_rsa" >> /etc/ssh/ssh_config

VOLUME [ "/root/.ssh" ]

EXPOSE 3000

CMD [ "launch" ]

Answer 22

I put together a very simple solution that works for my use case where I use a "builder" docker image to build an executable that gets deployed separately. In other words my "builder" image never leaves my local machine and only needs access to private repos/dependencies during the build phase.

You do not need to change your Dockerfile for this solution.

When you run your container, mount your ~/.ssh directory (this avoids having to bake the keys directly into the image, but rather ensures they're only available to a single container instance for a short period of time during the build phase). In my case I have several build scripts that automate my deployment.

Inside my build-and-package.sh script I run the container like this:

# do some script stuff before    

...

docker run --rm \
   -v ~/.ssh:/root/.ssh \
   -v "$workspace":/workspace \
   -w /workspace builder \
   bash -cl "./scripts/build-init.sh $executable"

...

# do some script stuff after (i.e. pull the built executable out of the workspace, etc.)

The build-init.sh script looks like this:

#!/bin/bash

set -eu

executable=$1

# start the ssh agent
eval $(ssh-agent) > /dev/null

# add the ssh key (ssh key should not have a passphrase)
ssh-add /root/.ssh/id_rsa

# execute the build command
swift build --product $executable -c release

So instead of executing the swift build command (or whatever build command is relevant to your environment) directly in the docker run command, we instead execute the build-init.sh script which starts the ssh-agent, then adds our ssh key to the agent, and finally executes our swift build command.

Note 1: For this to work you'll need to make sure your ssh key does not have a passphrase, otherwise the ssh-add /root/.ssh/id_rsa line will ask for a passphrase and interrupt the build script.

Note 2: Make sure you have the proper file permissions set on your script files so that they can be run.

Hopefully this provides a simple solution for others with a similar use case.

Answer 23

This is now available from 18.09 release!

According to the documentation:

The docker build has a --ssh option to allow the Docker Engine to forward SSH agent connections.

Here is an example of Dockerfile using SSH in the container:

# syntax=docker/dockerfile:experimental
FROM alpine

# Install ssh client and git
RUN apk add --no-cache openssh-client git

# Download public key for github.com
RUN mkdir -p -m 0600 ~/.ssh && ssh-keyscan github.com >> ~/.ssh/known_hosts

# Clone private repository
RUN --mount=type=ssh git clone [email protected]:myorg/myproject.git myproject

Once the Dockerfile is created, use the --ssh option for connectivity with the SSH agent:

$ docker build --ssh default .

Also, take a look at https://medium.com/@tonistiigi/build-secrets-and-ssh-forwarding-in-docker-18-09-ae8161d066

Answer 24

In later versions of docker (17.05) you can use multi stage builds. Which is the safest option as the previous builds can only ever be used by the subsequent build and are then destroyed

See the answer to my stackoverflow question for more info

Answer 25

I'm trying to work the problem the other way: adding public ssh key to an image. But in my trials, I discovered that "docker cp" is for copying FROM a container to a host. Item 3 in the answer by creak seems to be saying you can use docker cp to inject files into a container. See https://docs.docker.com/engine/reference/commandline/cp/

excerpt

Copy files/folders from a container's filesystem to the host path. Paths are relative to the root of the filesystem.

  Usage: docker cp CONTAINER:PATH HOSTPATH

  Copy files/folders from the PATH to the HOSTPATH

Answer 26

At first, some meta noise

There is a dangerously wrong advice in two highly upvoted answers here.

I commented, but since I have lost many days with this, please MIND:

Do not echo the private key into a file (meaning: echo "$ssh_prv_key" > /root/.ssh/id_ed25519). This will destroy the needed line format, at least in my case.

Use COPY or ADD instead. See Docker Load key “/root/.ssh/id_rsa”: invalid format for details.

This was also confirmed by another user:

I get Error loading key "/root/.ssh/id_ed25519": invalid format. Echo will remove newlines/tack on double quotes for me. Is this only for ubuntu or is there something different for alpine:3.10.3?

---

1. A working way that keeps the private key in the image (not so good!)

If the private key is stored in the image, you need to pay attention that you delete the public key from the git website, or that you do not publish the image. If you take care of this, this is secure. See below (2.) for a better way where you could also "forget to pay attention".

The Dockerfile looks as follows:

FROM ubuntu:latest
RUN apt-get update && apt-get install -y git
RUN mkdir -p /root/.ssh && chmod 700 /root/.ssh
COPY /.ssh/id_ed25519 /root/.ssh/id_ed25519
RUN chmod 600 /root/.ssh/id_ed25519 && \
    apt-get -yqq install openssh-client && \
    ssh-keyscan -t ed25519 -H gitlab.com >> /root/.ssh/known_hosts
RUN git clone [email protected]:GITLAB_USERNAME/test.git
RUN rm -r /root/.ssh

---

2. A working way that does not keep the private key in the image (good!)

The following is the more secure way of the same thing. If you need an image that has the git repo directory without the private key stored in one of its layers, you need two images, and you only use the second in the end. That means, you need FROM two times, and you can then copy only the git repo directory from the first to the second image, see the official guide "Use multi-stage builds".

We use "alpine" as the smallest possible base image which uses apk instead of apt-get; you can also use apt-get with the above code instead using FROM ubuntu:latest.

The Dockerfile looks as follows:

# first image only to download the git repo
FROM alpine as MY_TMP_GIT_IMAGE

RUN apk add --no-cache git
RUN mkdir -p /root/.ssh &&  chmod 700 /root/.ssh
COPY /.ssh/id_ed25519 /root/.ssh/id_ed25519
RUN chmod 600 /root/.ssh/id_ed25519

RUN apk -yqq add --no-cache openssh-client && ssh-keyscan -t ed25519 -H gitlab.com >> /root/.ssh/known_hosts
RUN git clone [email protected]:GITLAB_USERNAME/test.git
RUN rm -r /root/.ssh


# Start of the second image
FROM MY_BASE_IMAGE
COPY --from=MY_TMP_GIT_IMAGE /MY_GIT_REPO ./MY_GIT_REPO

We see here that FROM is just a namespace, it is like a header for the lines below it and can be addressed with an alias. Without an alias, --from=0 would be the first image (=FROM namespace).

You could now publish or share the second image, as the private key is not in its layers, and you would not necessarily need to remove the public key from the git website after one usage! Thus, you do not need to create a new key pair at every cloning of the repo. Of course, be aware that a passwordless private key is still insecure if someone might get a hand on your data in another way. If you are not sure about this, better remove the public key from the server after usage, and have a new key pair at every run.

---

A guide how to build the image from the Dockerfile

• Install Docker Desktop; or use docker inside WSL2 or Linux in a VirtualBox; or use docker in a standalone Linux partition / hard drive.

• Open a command prompt (PowerShell, terminal, ...).

• Go to the directory of the Dockerfile.

• Create a subfolder ".ssh/".

• For security reasons, create a new public and private SSH key pair - even if you already have another one lying around - for each Dockerfile run. In the command prompt, in your Dockerfile's folder, enter (mind, this overwrites without asking):

    Write-Output "y" | ssh-keygen -q -t ed25519 -f ./.ssh/id_ed25519 -N '""'
  

  (if you use PowerShell) or

    echo "y" | ssh-keygen -q -t ed25519 -f ./.ssh/id_ed25519 -N ''
  

  (if you do not use PowerShell).

  Your key pair will now be in the subfolder .ssh/. It is up to you whether you use that subfolder at all, you can also change the code to COPY id_ed25519 /root/.ssh/id_ed25519; then your private key needs to be in the Dockerfile's directory that you are in.

• Open the public key in an editor, copy the content and publish it to your server (e.g. GitHub / GitLab --> profile --> SSH keys). You can choose whatever name and end date. The final readable comment of the public key string (normally your computer name if you did not add a -C comment in the parameters of ssh-keygen) is not important, just leave it there.

• Start (Do not forget the "." at the end which is the build context):

  docker build -t test .

Only for 1.):

• After the run, remove the public key from the server (most important, and at best at once). The script removes the private key from the image, and you may also remove the private key from your local computer, since you should never use the key pair again. The reason: someone could get the private key from the image even if it was removed from the image. Quoting a user's comment:

  If anyone gets a hold of your image, they can retrieve the key... even if you delete that file in a later layer, b/c they can go back to Step 7 when you added it

  The attacker could wait with this private key until you use the key pair again.

Only for 2.):

• After the run, since the second image is the only image remaining after a build, we do not necessarily need to remove the key pair from client and host. We still have a small risk that the passwordless private key is taken from a local computer somewhere. That is why you may still remove the public key from the git server. You may also remove any stored private keys. But it is probably not needed in many projects where the main aim is rather to automate building the image, and less the security.

---

At last, some more meta noise

As to the dangerously wrong advice in the two highly upvoted answers here that use the problematic echo-of-the-private-key approach, here are the votes at the time of writing:

• https://stackoverflow.com/a/42125241/11154841 176 upvotes (top 1)
• https://stackoverflow.com/a/48565025/11154841 55 upvotes (top 5)
• While the question at 326k views, got a lot more: 376 upvotes

We see here that something must be wrong in the answers, as the top 1 answer votes are not at least on the level of the question votes.

There was just one small and unvoted comment at the end of the comment list of the top 1 answer naming the same echo-of-the-private-key problem (which is also quoted in this answer). And: that critical comment was made three years after the answer.

I have upvoted the top 1 answer myself. I only realised later that it would not work for me. Thus, swarm intelligence is working, but on a low flame? If anyone can explain to me why echoing the private key might work for others, but not for me, please comment. Else, 326k views (minus 2 comments ;) ) would have overseen or left aside the error of the top 1 answer. I would not write such a long text here if that echo-of-the-private-key code line would not have cost me many working days, with absolutely frustrating code picking from everything on the net.

Answer 27

You can pass the authorised keys in to your container using a shared folder and set permissions using a docker file like this:

FROM ubuntu:16.04
RUN apt-get install -y openssh-server
RUN mkdir /var/run/sshd
EXPOSE 22
RUN cp /root/auth/id_rsa.pub /root/.ssh/authorized_keys
RUN rm -f /root/auth
RUN chmod 700 /root/.ssh
RUN chmod 400 /root/.ssh/authorized_keys
RUN chown root. /root/.ssh/authorized_keys
CMD /usr/sbin/sshd -D

And your docker run contains something like the following to share an auth directory on the host (holding the authorised_keys) with the container then open up the ssh port which will be accessable through port 7001 on the host.

-d -v /home/thatsme/dockerfiles/auth:/root/auth -–publish=127.0.0.1:7001:22

You may want to look at https://github.com/jpetazzo/nsenter which appears to be another way to open a shell on a container and execute commands within a container.

Answer 28

Late to the party admittedly, how about this which will make your host operating system keys available to root inside the container, on the fly:

docker run -v ~/.ssh:/mnt -it my_image /bin/bash -c "ln -s /mnt /root/.ssh; ssh [email protected]"

I'm not in favour of using Dockerfile to install keys since iterations of your container may leave private keys behind.

Answer 29

In my case I had a problem with nodejs and 'npm i' from a remote repository. I fixed it added 'node' user to nodejs container and 700 to ~/.ssh in container.

Dockerfile:

USER node #added the part
COPY run.sh /usr/local/bin/
CMD ["run.sh"]

run.sh:

#!/bin/bash
chmod 700 -R ~/.ssh/; #added the part

docker-compose.yml:

nodejs:
      build: ./nodejs/10/
      container_name: nodejs
      restart: always
      ports:
        - "3000:3000"
      volumes:
        - ../www/:/var/www/html/:delegated
        - ./ssh:/home/node/.ssh #added the part
      links:
        - mailhog
      networks:
        - work-network

after that it started works

Answer 30

Here's how I did to use ssh key during image build using docker composer:

.env

SSH_PRIVATE_KEY=[base64 encoded sshkey]

docker-compose.yml

version: '3'
services:
  incatech_crawler:
    build:
      context: ./
      dockerfile: Dockerfile
      args:
        SSH_PRIVATE_KEY: ${SSH_PRIVATE_KEY} 

dockerfile: ...

# Set the working directory to /app
WORKDIR /usr/src/app/
ARG SSH_PRIVATE_KEY 
 
RUN mkdir /root/.ssh/  
RUN echo -n ${SSH_PRIVATE_KEY} | base64 --decode > /root/.ssh/id_rsa_wakay_user