Add sandbox Docker compose stack, project structure, and user manual

docker-compose.yml

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+version: "3.8"
+
+services:
+  postgres:
+    image: postgres:16-alpine
+    container_name: petmaster-postgres
+    restart: unless-stopped
+    environment:
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: postgres
+      POSTGRES_DB: petmaster_dev
+    ports:
+      - "5432:5432"
+    volumes:
+      - postgres-data:/var/lib/postgresql/data
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U postgres"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
+  redis:
+    image: redis:7-alpine
+    container_name: petmaster-redis
+    restart: unless-stopped
+    ports:
+      - "6379:6379"
+    volumes:
+      - redis-data:/data
+    healthcheck:
+      test: ["CMD", "redis-cli", "ping"]
+      interval: 10s
+      timeout: 5s
+      retries: 5
+
+  node-dev:
+    image: node:22-alpine
+    container_name: petmaster-node-dev
+    restart: unless-stopped
+    working_dir: /app
+    ports:
+      - "9229:9229"
+    volumes:
+      - ./node-data:/tmp/node-tmp
+    command: sh -c "while true; do sleep 3600; done"
+    depends_on:
+      postgres:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+
+  react-frontend:
+    image: node:22-alpine
+    container_name: petmaster-react-frontend
+    restart: unless-stopped
+    working_dir: /app
+    ports:
+      - "3000:3000"
+    volumes:
+      - ./node-data:/tmp/node-tmp
+    command: sh -c "while true; do sleep 3600; done"
+    depends_on:
+      postgres:
+        condition: service_healthy
+      redis:
+        condition: service_healthy
+
+volumes:
+  postgres-data:
+  redis-data: