GHA: Docker build caching and other speed improvements

[valentijnscholten]

TL;DR;
All tests in GitHub Actions have become a lot faster.

Integration tests before:

After:

Others are also a couple of minutes faster each.

Changes:

• Use Docker Layer Caching. If we only change some code, all the expensive build steps are cached (pip install -r requirements.txt etc)
• Build images in parallel and use artifact upload/download to pass on to the next job (in the same workflow)
• Use dummy image for initializer, celery(worker|beat) and nginx on unit tests
• Finetune Dockerfile.xxxx to make sure as much layers as possible can be cached
• Don't start any containers that are not needed for unit tests