Ensure all PRs to release to be tagged.

Will be enforced via branch protection.

.github/workflows/ensure-release-tagged.yaml

@@ -0,0 +1,26 @@
+name: Ensure Tagged Commits on Release
+
+on:
+  pull_request:
+    branches:
+      - release
+
+jobs:
+  check_tag:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+
+    - name: Check if base commit of PR is tagged
+      run: |
+        BASE_COMMIT=$(jq -r .pull_request.base.sha < "$GITHUB_EVENT_PATH")
+        TAG=$(git tag --contains $BASE_COMMIT)
+        if [ -z "$TAG" ]; then
+          echo "Base commit of PR is not tagged. PRs onto release must be tagged with the version number."
+          exit 1
+        fi
+        echo "Base commit of PR is tagged. Check passed."
+