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 Name Job ID Coverage
  Docker Image
passed debian-testing #113391

00:00:25

 
  Build And Test
passed build-and-test #113392

00:01:34