Add rubocop test stage

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>

.gitlab-ci.yml

 stages:
   - docker
+  - test
 
 .build-docker-image:
   stage: docker
@@ -41,7 +42,8 @@ frontend-base:
 frontend:
   extends: .build-docker-image
   needs:
-    - frontend-base
+    - job: frontend-base
+      optional: true
   variables:
     image: frontend
 
@@ -58,13 +60,30 @@ backend-base:
 backend:
   extends: .build-docker-image
   needs:
-    - backend-base
+    - job: backend-base
+      optional: true
   variables:
     image: backend
 
 worker:
   extends: .build-docker-image
   needs:
-    - backend-base
+    - job: backend-base
+      optional: true
   variables:
     image: worker
+
+rubocop:
+  stage: test
+  needs:
+    - job: frontend-base
+      optional: true
+  image:
+    name: $CI_REGISTRY_IMAGE/obs-frontend-base:$CI_COMMIT_REF_SLUG
+    entrypoint: [""]
+  script:
+    - cp -av /obs/src/api/Gemfile* /obs/src/api/.bundle src/api
+    - cd src/api &&
+      bundle config --local with development &&
+      bundle install --jobs=$(nproc) --retry=3 &&
+      bundle exec rake dev:lint:rubocop:rails