perf: use BuildKit cache mount for npm ci

Dockerfile

@@ -1,11 +1,14 @@
+# syntax=docker/dockerfile:1
 FROM node:20-alpine AS builder
 WORKDIR /app
 COPY package*.json ./
-RUN npm install
+RUN --mount=type=cache,target=/root/.npm \
+    npm ci --prefer-offline
 COPY . .
 ENV NEXTAUTH_SECRET=digital-home-jwt-secret-2026-cosmo
 ENV NEXTAUTH_URL=https://home.digital-home.site
-RUN npm run build
+RUN --mount=type=cache,target=/root/.npm \
+    npm run build
 
 FROM node:20-alpine AS runner
 WORKDIR /app