Docker follow symlink outside context

Question

Yet another Docker symlink question. I have a bunch of files that I want to copy over to all my Docker builds. My dir structure is:

parent_dir
    - common_files
        - file.txt
    - dir1
        - Dockerfile  
        - symlink -> ../common_files

In above example, I want file.txt to be copied over when I docker build inside dir1. But I don't want to maintain multiple copies of file.txt. Per this link, as of docker version 0.10, docker build must

Follow symlinks inside container's root for ADD build instructions.

But I get no such file or directory when I build with either of these lines in my Dockerfile:

ADD symlink /path/dirname or ADD symlink/file.txt /path/file.txt

mount option will NOT solve it for me (cross platform...). I tried tar -czh . | docker build -t without success.

Is there a way to make Docker follow the symlink and copy the common_files/file.txt into the built container?

Answer 1

That is not possible and will not be implemented. Please have a look at the discussion on github issue #1676:

We do not allow this because it's not repeatable. A symlink on your machine is the not the same as my machine and the same Dockerfile would produce two different results. Also having symlinks to /etc/paasswd would cause issues because it would link the host files and not your local files.

Answer 2

If anyone still has this issue I found a very nice solution on superuser.com:

https://superuser.com/questions/842642/how-to-make-a-symlinked-folder-appear-as-a-normal-folder

It basically suggests using tar to dereference the symlinks and feed the result into docker build:

$ tar -czh . | docker build -

Answer 3

One possibility is to run the build in the parent directory, with:

$ docker build [tags...] -f dir1/Dockerfile .

(Or equivalently, in child directory,)

$ docker build  [tags...] -f Dockerfile ..

The Dockerfile will have to be configured to do copy/add with appropriate paths. Depending on your setup, you might want a .dockerignore in the parent to leave out things you don't want to be put into the context.

Answer 4

instead of using simlinks it is possible to solve problem administratively by just moving files from sites_available to sites_enabled instead of copying or making simlinks

so your site config will be in one copy only in site_available folder if it stopped or something or in sites_enabled if it should be used

Answer 5

I know that it breaks portability of docker build, but you can use hard links instead of symbolic:

ln /some/file ./hardlink

Answer 6

I just had to solve this issue in the same context. My solution is to use hierarchical Docker builds. In other words:

parent_dir
  - common_files
    - Dockerfile
    - file.txt

- dir1
    - Dockerfile (FROM common_files:latest)

The disadvantage is that you have to remember to build common_files before dir1. The advantage is that if you have a number of dependant images then they are all a bit smaller due to using a common layer.