Ability to authenticate with external container registry

I need to use custom docker images for AWS Amplify builds. I tried publishing the image in following registries:
- Docker Hub
- GitHub Container Registry
- GitLab Container Registry

AWS Amplify does not seem to work with any of these as there is no way of authenticating with them:

Docker Hub [limits access for anonymous pulls](https://docs.docker.com/docker-hub/download-rate-limit/):

GitHub does not seem to allow access without `docker login` even for public images

GitLab - similar situation as with GitHub as I am unable to define a [token](https://docs.gitlab.com/ee/user/project/deploy_tokens/) in Amplify

**Describe the solution you'd like**
I'd like to see the ability to provide an [auth token](https://docs.github.com/en/free-pro-team@latest/packages/using-github-packages-with-your-projects-ecosystem/configuring-docker-for-use-with-github-packages) which would effectively be used to authenticate (`docker login`) with popular registries as suggested above.

**Additional context**
Given the above, the custom image feature is useless in most situations. In my case I need to use newer Node.js version as the default Amplify ones are still v12.

No repository files, tests, or entry points are identified in the issue. Start by locating the custom Docker image configuration and the AWS Amplify build authentication flow, then determine how registry credentials could be supplied for Docker Hub, GitHub Container Registry, and GitLab; done means authenticated pulls work for those registries.

Written by the indexing model from the issue text.