[CI] set-variables job should never be skipped : mandatory for other steps

.gitlab-ci.yml

@@ -62,12 +62,7 @@ set-variables:
     - ./get-modified-image.sh
   # If the Dockerfile is changed, force the build.
   # Otherwise, may be triggered manually.
-  rules:
-    - changes:
-      - "**/Dockerfile"
-      - "**/docker-compose.yml"
-      when: always
-    - when: manual
+  when: always
 
 # Build the image that was modified
 build: