Fix Docker build: use pnpm 11 (corepack) to match lockfile/allowBuilds schema

- pnpm-workspace.yaml from Astro v7 uses allowBuilds (pnpm 10.14+ schema);
  pnpm 9 in old Dockerfile errored 'packages field missing or empty'
- Switch Dockerfile to corepack + pnpm@11, matching pnpm-lock.yaml
- Verified pnpm install --frozen-lockfile passes with pnpm 11

Dockerfile

@@ -1,6 +1,6 @@
 # =============================================================================
 # มิตซูชัยพร สมุทรสาคร — Astro static site
-# Multi-stage: 1) build with node/pnpm 2) serve dist with nginx.
+# Multi-stage: 1) build with node + pnpm 2) serve dist with nginx.
 #
 # Env vars (GA_ID, FORM_EMAIL, GMAP_LAT/LNG) are injected into HTML by
 # entrypoint.sh at container startup (see .env.example).
@@ -11,8 +11,9 @@
 FROM node:22-alpine AS build
 WORKDIR /app
 
-# pnpm
-RUN npm install -g pnpm@9
+# pnpm via corepack (uses the version pinned by packageManager / pnpm-workspace)
+# Use pnpm 11 to match the pnpm-lock.yaml + allowBuilds schema from Astro v7.
+RUN corepack enable && corepack prepare pnpm@11 --activate
 
 # Install deps (uses pnpm-lock.yaml)
 COPY package.json pnpm-lock.yaml pnpm-workspace.yaml ./