[ci] Add MR pipelines.

.gitlab-ci.yml

+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_COMMIT_TAG
+    - if: $CI_PIPELINE_SOURCE == 'web'
+    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
+
 stages:
   - build
   - release