gitlab-ci: Don't rely on $CI_PROJECT_NAME

The name of a forked repository can be changed later, in which case this
would fail to refer to the main repository.

Pointed out by Eric Engestrom in
mesa/mesa!224 .
2 jobs for gitlab-ci-docker-image-only in 1 minute and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #113391
debian-testing

00:00:25

 
  Build And Test
passed #113392
build-and-test

00:01:34