Add container readiness checks and exclude local state from images

Adapted from MikaelKW/nodecast-tv-plus readiness and container context work.

Co-authored-by: MikaelKW <87399352+MikaelKW@users.noreply.github.com>

.github/workflows/container.yml

@@ -0,0 +1,39 @@
+name: Container Smoke Test
+
+on:
+  pull_request:
+    paths: [Dockerfile, .dockerignore, 'server/**', scripts/healthcheck.cjs, 'package*.json', .github/workflows/container.yml]
+
+permissions:
+  contents: read
+
+jobs:
+  container:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v7
+      - name: Build and check readiness
+        shell: bash
+        run: |
+          set -euo pipefail
+          mkdir -p data
+          printf 'build context sentinel' > .env.container-sentinel
+          printf 'build context sentinel' > data/container-sentinel
+          docker build -t nodecast-smoke .
+          docker run -d --name nodecast-smoke nodecast-smoke
+          for attempt in $(seq 1 60); do
+            if docker exec nodecast-smoke node scripts/healthcheck.cjs; then
+              docker exec nodecast-smoke sh -c 'test ! -e /app/.env.container-sentinel && test ! -e /app/data/container-sentinel'
+              exit 0
+            fi
+            sleep 2
+          done
+          docker logs nodecast-smoke
+          exit 1
+      - name: Show container logs on failure
+        if: failure()
+        run: docker logs nodecast-smoke || true
+      - name: Remove smoke container
+        if: always()
+        run: docker rm -f nodecast-smoke || true