Change only to rules block

Because only is a deprecated feature

.gitlab-ci.yml

@@ -6,9 +6,8 @@ build-vue:
   script:
     - cd frontend
     - docker build --pull -t vue .
-  only:
-    - merge_requests
-
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
 
 build-backend:
   image: docker:latest
@@ -18,13 +17,13 @@ build-backend:
   script:
     - cd backend
     - docker build --pull -t backend .
-  only:
-    - merge_requests
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
 
 build-android:
   image: debian
   stage: build
   script: 
     - echo "To be done"
-  only: 
-    - merge_requests
+  rules:
+    - if: $CI_MERGE_REQUEST_ID