Add .gitlab-ci.yml

Builds jhbuild and tests the default module set. The current build result can be found at https://gitlab.gnome.org/krichter/jhbuild/pipelines/33232. There're the following issues:

• jhbuild bootstrap fails due to jhbuild bootstrap: Repository=gnome.org not found for module id=m4-common. Possible repositories are ragel, igj, sqlite, expat, cmake, launchpad, gnome-http, python, cairo, sourceware, libproxy, pkgconfig, tango, ftp.gnu.org, tukaani.org, git.gnome.org, xorg, sourceforge, xmlsoft, dbus which is probably a bug
• jhbuild build --nodeps fails due to *** Error during phase configure of gtk-doc: ########## Error running meson --prefix /root/jhbuild/install --libdir lib --buildtype=debugoptimized /root/jhbuild/checkout/gtk-doc *** [7/208]

I'm grateful for advice how to fix these issues.