ci: build release targets on independent matrix runners

.github/workflows/backend-ci.yml

@@ -80,3 +80,14 @@ jobs:
           version: v2.13
           args: --timeout=30m
           working-directory: backend
+
+  release-helpers:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - uses: actions/setup-python@v6
+        with:
+          python-version: '3.12'
+      - run: python -m pip install -r .github/release-tools/requirements-release.txt
+      - run: python -m unittest discover -s .github/release -p 'test_release_matrix.py'
+      - run: bash -n .github/release-tools/release-images.sh