CI: Hook up GitLab CI

The main tasks it does is build on a few different distros as well as build
with the various build options to make sure they work.It doesn't (yet) run the
test suite runner because that one mostly requires device nodes to operate on.

Most of the fancy is to get the docker images ready. A dnf update takes
forever, so we don't want to do that on 10 different machines. So instead we
build docker images with all the bits pre-installed, push that to the registry
and use those images for testing.

To speed things up, we only do that when the current image is older than a
week. And we only do that when we push to libinput proper, so a merge request
or pushing to your private gitlab repo will never trigger a docker image
update - it will trigger the tests and use the docker images tough.

Co-authored-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
(cherry picked from commit 8f3a8a5)