Only run when dockerfile has changed

.gitlab-ci.yml

@@ -33,6 +33,8 @@ docker-build:
     - docker push "$CI_REGISTRY_IMAGE${tag}"
   # Run this job in a branch where a Dockerfile exists
   rules:
+    - changes:
+      - Dockerfile
     - if: $CI_COMMIT_BRANCH
       exists:
         - Dockerfile