[ci] pass down upstream mr branch name

.gitlab-ci.yml

@@ -7,6 +7,8 @@ variables:
 # rules for the default triggers:
 # - pipelines are triggered for commits to master, tags, merge requests, external triggers
 # - Within merge requests, we require to start the pipeline manually by clicking play for the trigger
+# - if the pipeline was triggered externally (from dumux), we expect the variable DUMUX_MERGE_REQUEST_BRANCH
+#   and pass it to this project's test pipeline as DUMUX_BRANCH variable
 .default-trigger:
   stage: trigger pipelines
   trigger:
@@ -18,6 +20,8 @@ variables:
     - if: $CI_PIPELINE_SOURCE == "pipeline"
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
       when: manual
+  variables:
+    DUMUX_BRANCH: $DUMUX_MERGE_REQUEST_BRANCH
 
 full-dune-2.7-gcc:
   extends: .default-trigger