build(docker): use Chinese mirrors for backend dependencies

backend/Dockerfile

@@ -2,6 +2,12 @@
 
 FROM python:3.10-slim
 
+# Point Debian and pip at Chinese mirrors reachable from the CI runner.
+RUN find /etc/apt -type f \( -name '*.sources' -o -name '*.list' \) \
+    -exec sed -i 's|http://deb.debian.org|https://mirrors.tuna.tsinghua.edu.cn|g' {} +
+
+ENV PIP_INDEX_URL=https://pypi.tuna.tsinghua.edu.cn/simple
+
 # Install build tools needed for the C++ extension plus curl for healthchecks
 RUN apt-get update && apt-get install -y --no-install-recommends \
     build-essential \