ci: run pipeline only on merge request events

.gitlab-ci.yml

@@ -1,6 +1,12 @@
 # vim: set ts=4 sw=4 expandtab:
 image: ubuntu-dramsys
 
+workflow:
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+      when: always
+    - when: never
+
 stages:
     - build
     - test