ci: add validation workflow — compose config, nginx syntax, GHCR image check

Author: rmdes

.github/workflows/validate.yml

@@ -0,0 +1,27 @@
+name: Validate docker-compose
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+env:
+  APP_IMAGE: ghcr.io/rmdes/newsdiff:main
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Validate docker-compose.yml
+        run: docker compose config --quiet
+
+      - name: Check nginx config syntax
+        run: docker run --rm -v "$PWD/nginx.conf:/etc/nginx/nginx.conf:ro" nginx:alpine nginx -t
+
+      - name: Verify GHCR image exists
+        run: |
+          echo "Checking $APP_IMAGE..."
+          docker manifest inspect "$APP_IMAGE" > /dev/null 2>&1 && echo "Image exists" || echo "::warning::Image not found on GHCR"