.gitlab-ci.yml: Enable MR pipelines

This should improve fork-based contributions.

Signed-off-by: Felix Walter <felix.walter@d3tn.com>

.gitlab-ci.yml

@@ -10,6 +10,14 @@ image: registry.gitlab.com/d3tn/ud3tn-docker-images/ci-python-clang:3.9-bookworm
 variables:
   GIT_SUBMODULE_STRATEGY: recursive
 
+# https://docs.gitlab.com/ci/yaml/workflow/#switch-between-branch-pipelines-and-merge-request-pipelines
+workflow:
+    rules:
+        - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+        - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+          when: never
+        - if: $CI_COMMIT_BRANCH
+
 build-posix-debug:
     stage: builds
     when: always