This adds compilation testing using meson on the gitlab instance. It
uses a prebuild image built, described in .gitlab-ci/Dockerfile, based
on Fedora 29.
The image is build and published by running:
cd .gitlab-ci/
docker build -t registry.gitlab.gnome.org/gnome/mutter/master:v1 .
docker push registry.gitlab.gnome.org/gnome/mutter/master:v1
Resolves: https://gitlab.gnome.org/GNOME/mutter/merge_requests/132
