优化Dockerfile，复用前端构建阶段的依赖

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

Dockerfile

@@ -19,18 +19,14 @@ RUN pnpm install --no-frozen-lockfile
 RUN pnpm build
 
 # ===== 后端构建阶段 =====
-FROM node:20-alpine AS backend-builder
+FROM frontend-builder AS backend-builder
 
 WORKDIR /app
 
-# 复制后端代码和配置
-COPY apps/backend ./apps/backend
-COPY package.json pnpm-workspace.yaml ./
-
-# 安装pnpm和依赖
-RUN npm install -g pnpm@9 && \
-    cd apps/backend && \
-    pnpm install --no-frozen-lockfile
+# 后端依赖已经在前端构建阶段安装完成
+# 构建后端（如果需要）
+WORKDIR /app/apps/backend
+RUN pnpm build || echo "No build script or build not needed"
 
 # ===== Nginx + Node.js 运行阶段 =====
 FROM node:20-alpine AS runner