How to deploy a React SSR app that doesn't use Next.js to AWS Amplify?

Question

AWS released SSR support on May 18th, 2021: https://aws.amazon.com/about-aws/whats-new/2021/05/aws-amplify-hosting-announces-server-side-rendering-support-for-next-js-web-apps/

I've been able to deploy a Next.js app without any issues, however, when attempting to deploy a custom React SSR app, I'm not sure how to do it, and there doesn't appear to be any documentation / blogs about it... is it even possible?

For example, here is my ampify.yml file:

version: 1
frontend:
  phases:
    preBuild:
      commands:
        - yarn install
    build:
      commands:
        - yarn run build
  artifacts:
    baseDirectory: dist
    files:
      - '**/*'
  cache:
    paths:
      - node_modules/**/*

Here is the relevant scripts from my package.json file:

  "scripts": {
    "prod::ssr": "cross-env NODE_ENV=production MODE=ssr webpack",
    "prod::client": "cross-env NODE_ENV=production MODE=client webpack",
    "build": "npm run prod::ssr && npm run prod::client",
    "start": "node dist/ssr/index.js"
  },

This works fine locally, but not on AWS Ampify.

My understanding is AWS Ampify uses the start command to run the app... is this true?