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.