Consider using GitHub Actions to build Docker images

[mikegerber]

GitHub Actions is free for public repositories (unlike CircleCI and Travis CI).

• Branch test/github-actions
• Build based on matrix
• Dynamically create build matrix
• Tag images based on branch IF all builds were successful
• Correct tags for branch/release tags
• It should be possible to run edge (or any other tag) instead of latest
• Processors should work with the default models/resources #46
• Test built container images
• Rename DRONE_COMMIT
• Remove build on test branch again

[mikegerber]

I'd prefer using this over a private Drone CI installation or even Jenkins. Reasons:

• Jenkins does not spark any joy at all.
• I do not want to maintain a Drone CI installation if not necessary.
• I also would like to have the CPU/IO load at GitHub
• Drone CI has some shortcoming/inflexibilites
• GitHub Actions was really great to use in another project, I just need to see if it can cope with this multiple-image build
• I'd much prefer a transparent build process (i.e. people can audit the logs) over a private build

[mikegerber]

Possibly useful info w.r.t. building multiple images:

• How build and push multiple docker image with same repo and same version, but different name? docker/build-push-action#561
• https://stackoverflow.com/questions/67475914/github-action-run-one-step-before-matrix-combinations
• https://github.com/actions/upload-artifact

[mikegerber]

This might make it possible to only tag all images latest (#64) if all images are built successfully:
https://stackoverflow.com/questions/59180385/using-an-array-of-values-to-repeat-a-step-in-github-actions-workflow

Pseudo code:

• fail if anything is wrong ("set -e")
• build image
  for image in images
• tag image latest
  for image in images

[mikegerber]

Finally all builds work in the test branch:

https://github.com/qurator-spk/ocrd-galley/actions/runs/4145656085

This just needs some polishing (setting the correct tags for a release), testing and then prebuilt images are finally usable again.

[mikegerber]

* [x]  It should be possible to run `edge` (or any other tag) instead of `latest`

Yeah, it's done by setting DOCKER_IMAGE_TAG and the build already uses it, I just forgot.

[mikegerber]

* [ ]  Tag images based on branch IF all builds were successful

Done in test/github-actions!

[mikegerber]

* [ ]  Correct tags for branch/release tags

Our config of docker's meta-data-action should do this fine:

        type=ref,event=branch
        type=semver,pattern={{version}}

I've only tested the event=branch part, but the semver version tags should work fine.

[mikegerber]

* [ ]  Test built container images

We currently just 1. call a few processor --versions, and 2. tested manually. This is not perfect, but for now this suffices.

[mikegerber]

* [ ]  Rename `DRONE_COMMIT`

Now using GIT_COMMIT. It uses the sha- prefix from metadata-action but as I think most users wouldn't use it anyway and we are the main users ourselves (used in the build), that's alright.

[mikegerber]

#46 is now solved in a minimal way to make these images work. Going to add some documentation and then this is ready to be merged!!

[mikegerber]

This is now merged!