Maintenance: Improve docker CI configuration

.gitlab/ci/test/docker.yml

@@ -3,7 +3,7 @@ docker:
   stage: test
   interruptible: false
   rules:
-    - if: $CI_COMMIT_REF_NAME =~ /^private/
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event" && $CI_COMMIT_REF_NAME =~ /^private/
       when: manual
       allow_failure: true
     - when: on_success