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.