Fix: PIP change to China mirror

Dockerfile

@@ -26,7 +26,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
 
 # Install Python dependencies
 COPY backend/requirements.txt ./
-RUN pip install --no-cache-dir -r requirements.txt
+RUN pip -i https://pypi.tuna.tsinghua.edu.cn/simple install --no-cache-dir -r requirements.txt
 
 # Copy backend code
 COPY backend/ ./