Make this repo work as a fork

It might be a tall order, but it would be nice if the CI would work in this project when someone forks it. At the moment we're stuck using branches for development mainly (in my naive view) because:

• we require authentication to access upstream code. This shouldn't really be required though because everything we're building from is now public.
• we require authentication to push images to dockerhub. Maybe we can get around this by pushing the images to the gitlab registry at CERN instead. Or maybe not.
• there are other repo-specific things that have to be configured. At the very least it would be nice to document what these things are, but more optimistically maybe they could be eliminated.