Mutter's CI now also builds gnome-shell to ensure that a MR doesn't
break the shell. Its docker image has therefore been updated to contain
all our deps as well, so we don't need our own image anymore.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/546
So far we are only performing a basic syntax check on javascript
sources; it's time to test the C code as well. As mutter is tightly
coupled, we bite the bullet and build it as well, either using a
matching branch (if it exists), or current master.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/408