ci: Add GitLab CI description file

ci: Add GitLab CI description file

We're mostly interested into building and testing everything that gets
pushed to the repository — including merge requests.

When pushing tags, though, we should assume we're spinning a release, so
let's run the dist target, and store the tarball, and the generated
documentation while we're at it, as artifacts on GitLab.

The Dockerfile for the image used for the build is included in tree, and
published on Docker Hub. Using a custom image allows us to avoid the
costly "download and install build dependencies" phase, as well as
controlling the environment a little bit better.

https://bugzilla.gnome.org/show_bug.cgi?id=793635