Workflow: Cleanup after PR (#1821)

* Stubbing out new workflow to test logic in feature branch

* fixing trigger syntax

.github/workflows/cleanup-after-pr.yml

@@ -0,0 +1,14 @@
+---
+name: Clean After PR
+
+on:
+  pull_request:
+    types: [closed]
+
+jobs:
+  build-docker:
+    name: Remove feature branch docker images
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Stub
+        run: echo "This is a workflow stub"