[CI] Use a common setup
Put most stuff into ci-main.yml, and include that from .gitlab-ci.yml. This means I can automatically verify the common stuff is up-to-date.
Adresses: https://gitlab.dune-project.org/joe/pacxx-docker/issues/25
WIP: Waiting for https://gitlab.dune-project.org/joe/pacxx-docker/merge_requests/61 to hit master
Edited by Dr. Jorrit Fahlke