fix(security): complete security and delivery compliance remediation

backend/Dockerfile

@@ -1,13 +1,12 @@
-FROM python:3.11-slim
+ARG PYTHON_BASE_IMAGE=python:3.11-slim
+FROM ${PYTHON_BASE_IMAGE}
 
 WORKDIR /app
 
-# 使用清华镜像源
-RUN pip config set global.index-url https://pypi.tuna.tsinghua.edu.cn/simple && \
-    pip config set global.trusted-host https://pypi.tuna.tsinghua.edu.cn
-
 COPY requirements.txt .
-RUN pip install --no-cache-dir -r requirements.txt
+ARG PIP_INDEX_URL
+RUN if [ -n "$PIP_INDEX_URL" ]; then pip config set global.index-url "$PIP_INDEX_URL"; fi \
+    && pip install --no-cache-dir -r requirements.txt
 
 COPY . .