gitlab CI: add a basic build test

Based on debian stable because we don't expect this repo to be updated very
often, so let's not play catch-up with more frequently updated distributions.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
2 jobs for master in 1 minute and 35 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Prep
passed #5860833
container-prep

00:01:06

 
  Build
passed #5860834
build

00:00:28