fix: 更新 Dockerfile.backend 配置

Co-Authored-By: Claude <noreply@anthropic.com>

docker/app/Dockerfile.backend

@@ -7,8 +7,8 @@ ENV PYTHONDONTWRITEBYTECODE=1 \
 WORKDIR /app
 
 COPY backend/requirements.txt /tmp/requirements.txt
-RUN pip install --upgrade pip \
-    && pip install -r /tmp/requirements.txt \
+RUN pip install --upgrade pip -i https://pypi.tuna.tsinghua.edu.cn/simple \
+    && pip install -r /tmp/requirements.txt -i https://pypi.tuna.tsinghua.edu.cn/simple \
     && rm -f /tmp/requirements.txt
 
 RUN python -c "import fastapi, uvicorn, psycopg, sqlalchemy, redis, jwt, passlib, httpx, alembic; print('backend dependency check ok')"