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 #147561
debian-testing

00:00:24

 
  Build
passed #147564
xserver-1.13

00:00:43

passed #147565
xserver-1.14

00:00:42

passed #147566
xserver-1.15

00:00:45

passed #147567
xserver-1.16

00:00:25

passed #147568
xserver-1.17

00:00:46

passed #147569
xserver-1.18

00:00:47

passed #147570
xserver-1.18-clang

00:00:26

passed #147571
xserver-1.19

00:00:45

passed #147572
xserver-1.19-clang

00:00:47

passed #147562
xserver-1.20

00:00:31

passed #147563
xserver-1.20-clang

00:00:28