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

Otherwise there's normally no need to run it. It will also run when a
new branch is created, which ensures that the docker image always exists
(e.g. in a newly forked repository).

Inspired by https://gitlab.freedesktop.org/mesa/mesa/merge_requests/143

.gitlab-ci.yml

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