renovate: add workflow to validate config

.github/workflows/renovate-validate.yaml

@@ -0,0 +1,12 @@
+name: Validate renovate config
+on:
+  pull_request: {}
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608 # v4.1.0
+      - run: |-
+          docker run -i --rm -v $(realpath .):/repo -w /repo --entrypoint renovate-config-validator ghcr.io/renovatebot/renovate --strict