feat(docker): add multi-stage image and compose with persistent SQLite

Mount ./docker-data for WAL-capable SQLite storage, support DATABASE_PATH alias, and document compose secrets via .env.docker.example.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker-compose.yml

@@ -0,0 +1,18 @@
+services:
+  backend:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    container_name: child-gold-land-backend
+    ports:
+      - "3000:3000"
+    env_file:
+      - .env.docker
+    environment:
+      NODE_ENV: production
+      PORT: 3000
+      HOST: 0.0.0.0
+      DB_PATH: /app/data/app.db
+    volumes:
+      - ./docker-data:/app/data
+    restart: unless-stopped