后端镜像 apk 改用阿里云 Alpine 源，加速生产构建

国内机器安装 tzdata 时访问官方 dl-cdn 过慢，构建阶段切换为 mirrors.aliyun.com。

backend/Dockerfile

@@ -21,7 +21,10 @@ FROM alpine:3.22
 
 WORKDIR /app
 ENV TZ=Asia/Shanghai
-RUN apk add --no-cache tzdata
+# 国内服务器访问 dl-cdn.alpinelinux.org 常极慢，改用阿里云镜像
+RUN sed -i 's#https\?://dl-cdn.alpinelinux.org/alpine#https://mirrors.aliyun.com/alpine#g' \
+      /etc/apk/repositories \
+    && apk add --no-cache tzdata
 COPY --from=build /out/hfb-api /app/hfb-api
 COPY --from=build /go/bin/goose /app/goose
 COPY --from=build /src/migrations /app/migrations