introduce manual action and automatic action with rule

.gitlab-ci.yml

@@ -28,11 +28,18 @@ test:
     - make check
 
 build:varnish:
-  stage: build
-  image: docker:19.03.8
+  extends: .build-varnish
   rules:
     - changes:
       - container/Dockerfile.varnish
+
+build:varnish:manual:
+  extends: .build-varnish
+  when: manual
+
+.build-varnish:
+  stage: build
+  image: docker:19.03.8
     - when: manual
   services:
     - docker:19.03.8-dind