gitlab CI: add a basic build test

Build an Arch image that tests a meson build with all build options we support
(well, the single one so far).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
3 jobs for wip/gitlab-ci in 38 seconds (queued for 1 second)
Status Name Job ID Coverage
  Prep
passed container-prep #6770324

00:00:11

 
  Build
passed meson: [] #6770325

00:00:10

passed meson: [-Dlegacy=true] #6770326

00:00:27