ci: 添加国内镜像代理和 BuildKit 缓存优化构建速度

frontend/Dockerfile

@@ -3,6 +3,9 @@ FROM node:22-alpine AS builder
 
 WORKDIR /app
 
+# 使用国内 npm 镜像
+RUN npm config set registry https://registry.npmmirror.com
+
 # 先复制依赖清单，利用 Docker 缓存层
 COPY package.json package-lock.json ./
 RUN npm ci