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

This reverts commit 8694395fcfd407190db74f31064960e484747377.

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.
(Cherry picked from commit 0ec9a13c2a207e98e2dd6b352552c0e069af0727)

Author: Michel Dänzer

.gitlab-ci.yml

@@ -23,10 +23,6 @@ stages:
 
 debian-testing:
     stage: docker-image
-    only:
-        changes:
-            - .gitlab-ci.yml
-            - .gitlab-ci/Dockerfile
     image:
         name: gcr.io/kaniko-project/executor:debug
         entrypoint: [""]