Speed up Gitlab CI runs by using pre-build images

We currently spend quite some time during each CI job on downloading and unpacking packages from the Debian repositories. We can speed this up by pre-building Docker images that contains all the packages we need, and take the additional cost in larger Docker images, but with less things to do in each run.

Ideally, we should make sure that we also automatically re-build these images once a week or monthly to ensure that the Debian Testing based images remain up-to-date and that the Debian Stable images gets the security updates that users are also receiving.