Build assets from the MR HEAD commit instead of the merge result commit

Signed-off-by: Rémy Coutable <remy@rymai.me>

.gitlab/ci/frontend.gitlab-ci.yml

@@ -39,6 +39,15 @@ compile-production-assets:
       - public/assets/
       - webpack-report/
     when: always
+  before_script:
+    - if [ -n "$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA" ]; then
+        echo "Checking out \$CI_MERGE_REQUEST_SOURCE_BRANCH_SHA ($CI_MERGE_REQUEST_SOURCE_BRANCH_SHA) instead of \$CI_COMMIT_SHA (merge result commit $CI_COMMIT_SHA) so that GitLab assets image tag actually reflect the commit for which assets were compiled.";
+        git checkout -f ${CI_MERGE_REQUEST_SOURCE_BRANCH_SHA};
+      else
+        echo "Building the image from \$CI_COMMIT_SHA ($CI_COMMIT_SHA) for this non-merge result pipeline.";
+      fi;
+    - echo "See https://docs.gitlab.com/ee/development/testing_guide/end_to_end/index.html#with-pipeline-for-merged-results for more details.";
+    - !reference [.default-before_script, before_script]
   after_script:
     - rm -f /etc/apt/sources.list.d/google*.list  # We don't need to update Chrome here