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

This reverts commit 8694395f.

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.
4 jobs for revert-gitlab-ci-only in 9 minutes and 18 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Docker Image
passed #143652
debian-testing

00:00:37

 
  Build And Test
passed #143653
autotools-build-and-test

00:06:43

passed #143655
meson-build-and-test

00:01:57

failed #143654
meson-build-and-test

00:01:09