workflows: build xenial-base using Github actions

[julianoes]

Let's try to use this instead of building on dockerhub.

[TSC21]

@julianoes is there a specific reason we are moving to Github Actions?

[julianoes]

@TSC21 I just wanted to experiment if that could be faster and more transparent. The way via dockerhub is slow and hard to understand.

[julianoes]

This now works for the two base images. The question is how this could work for images on top of the base image. The problem is that we reference the tag in the Dockerfile but that tag has not been built yet. So essentially we can't reference thebase in the pull request as it will always fail. We could reference to latest and push to latest in a PR but then different PRs would conflict.

An alternative would be not to build anything in the PR and then build it all using latest once merged to master.
Another way would be to only build on tag (like dockerhub), and at that point correctly follow the tags by building in the right order.

@TSC21 @dagar better ideas?

[TSC21]

@julianoes I don't really like the idea of using the latest tag, but if there's no other solution, we might have to use it.

[julianoes]

@TSC21 I lean towards building everything on master but not in the pull request.