ci: cache pnpm packages during image builds

Dockerfile

@@ -1,14 +1,21 @@
+# syntax=docker/dockerfile:1.7
+
 # 构建 Vue 静态文件
 FROM node:22-alpine AS build
 
 WORKDIR /app
 
 # Corepack 随 Node 一起提供，用它固定使用项目的 pnpm。
+ENV PNPM_HOME=/pnpm
+ENV PATH=$PNPM_HOME:$PATH
 RUN corepack enable
 
 COPY package.json pnpm-lock.yaml ./
 COPY shared/package.json ./shared/package.json
-RUN pnpm install --frozen-lockfile
+# pnpm store 使用 BuildKit 持久缓存；依赖未变化时不会重新下载。
+RUN --mount=type=cache,id=admin-web-pnpm-store,target=/pnpm/store \
+    pnpm config set store-dir /pnpm/store && \
+    pnpm install --frozen-lockfile
 
 COPY . .
 RUN pnpm test && pnpm build