.gitlab-ci.yml: Add minimal CI
This will run only for the master branch at the moment (post commit)
We want to eventually have a pre-commit CI, but currently this would take too much time as we will have to build almost everything every time, as we are following master of everything
As this is only going to work for master, you have this as it whould work in a test pipeline here:
Fixes #11 (closed)
Edited by Javier Jardón