ci: Add Gitlab-CI configuration

ci: Add Gitlab-CI configuration

This uses the same shell scripts as Travis-CI, with slightly different
settings. We use Docker containers for all our Gitlab-CI runs, so take
the opportunity to use Debian 9 'stretch' as our baseline, and
relegate Ubuntu 14.04 'trusty' to to a secondary build.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=108177
Acked-by: Philip Withnall <withnall@endlessm.com>
(cherry picked from commit 60933c09e9e891f74f0102fabe22d29a1a7ae5c5)