GitLab

Migrate to mirroring through Gitlab directly

No '-' suffix for non-default branches

In base Ubuntu image? Without this recent Let's Encrypt certs are not
accepted

- Determine dockertag based on branch name, to simplify structure
- Push to Hub instead of triggering a build (which is now behing a
paywall)

This keeps the chain as it was (now that a devel user is added to the
base image).

- With build trigger to Docker Hub
- Rename image to use Docker Hub's namespace scheme