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 .
13 jobs for gitlab-ci-docker-image-only in 3 minutes and 4 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #110215

00:00:21

failed debian-testing #108464

01:00:00

 
  Build
passed xserver-1.13 #108467

00:00:24

passed xserver-1.14 #108468

00:00:25

passed xserver-1.15 #108469

00:00:24

passed xserver-1.16 #108470

00:00:24

passed xserver-1.17 #108471

00:00:25

passed xserver-1.18 #108472

00:00:25

passed xserver-1.18-clang #108473

00:01:08

passed xserver-1.19 #108474

00:00:24

passed xserver-1.19-clang #108475

00:00:26

passed xserver-1.20 #108465

00:00:32

passed xserver-1.20-clang #108466

00:00:31