Move all build environment images to Github Container Registry

[FooBarWidget]

When we add/change a build environment image (e.g. when we're adding support for a new distro), the image must currently be manually pushed to the Docker Hub. This requires authorization to Docker Hub. But we don't want the pipeline to have admin access to all images under fullstaq/.

We should migrate to Github Container Registry which allows authorization on a per-image basis.

[PaulusTM]

With the new GitHub Container Registry available (https://github.blog/2020-09-01-introducing-github-container-registry/) it might be a good idea to put all images there as it it linked to the repo.

If you like this idea I'll give this a shot.

[FooBarWidget]

Yes I'd appreciate that.

[FooBarWidget]

All images have been moved to Github Container Registry. But we don't auto-publish yet.