Update Dockerfile

Dockerfile

@@ -14,11 +14,11 @@ RUN npm run build
 
 # 阶段 2：运行后端 + 提供前端静态资源
 FROM python:3.9-slim
-WORKDIR /app
+WORKDIR /app/backend
 
 # 后端依赖
-COPY backend/requirements.txt ./backend/requirements.txt
-RUN pip install --no-cache-dir -r backend/requirements.txt
+COPY backend/requirements.txt ./
+RUN pip install -r backend/requirements.txt
 
 # 拷贝后端代码
 COPY backend/ ./backend/