Add a label forcing a test container rebuild on PRs (#2180) (#2186)

Usually, the test container only needs rebuilding when a change to the
tests or related files happen. However, when creating PRs that target
old branches (like when backporting changes to a release branch), the
rebuild may not trigger and an older image may be subject to newer tests
it is not meant to pass.

In order to work around this, a label for triggering rebuilds of the
test container manually is added, allowing PRs to rebuild the test
container on demand.

Author: Molter73

.github/workflows/integration-test-containers.yml

@@ -55,7 +55,11 @@ jobs:
     needs:
       - should-build-test-image
     if: |
-      (github.event_name != 'pull_request' || needs.should-build-test-image.outputs.build-image == 'true') &&
+      (
+        github.event_name != 'pull_request' ||
+        needs.should-build-test-image.outputs.build-image == 'true' ||
+        contains(github.event.pull_request.labels.*.name, 'rebuild-test-container')
+      ) &&
       !contains(github.event.pull_request.labels.*.name, 'skip-integration-tests')
 
     outputs: