Revert "gitlab-ci: Only run docker-image stage if relevant source files change"

This reverts commit 9c23076b.

Some scenarios have come to light where this failed to ensure the docker
image exists:

* If the master branch of a forked repository is used for an MR which
  doesn't modify .gitlab-ci.yml, the docker-image job may not run.
* If the docker-image job of the first pipeline in a forked repository
  is cancelled or fails for any reason, and .gitlab-ci.yml isn't
  modified for the next pipeline run.
12 jobs for revert-gitlab-ci-only in 1 minute and 12 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed debian-testing #147561

00:00:24

 
  Build
passed xserver-1.13 #147564

00:00:43

passed xserver-1.14 #147565

00:00:42

passed xserver-1.15 #147566

00:00:45

passed xserver-1.16 #147567

00:00:25

passed xserver-1.17 #147568

00:00:46

passed xserver-1.18 #147569

00:00:47

passed xserver-1.18-clang #147570

00:00:26

passed xserver-1.19 #147571

00:00:45

passed xserver-1.19-clang #147572

00:00:47

passed xserver-1.20 #147562

00:00:31

passed xserver-1.20-clang #147563

00:00:28