use buildkit cache mounts for npm and next build cache

Dockerfile

@@ -4,13 +4,15 @@ FROM base AS deps
 RUN apk add --no-cache libc6-compat
 WORKDIR /app
 COPY package.json package-lock.json ./
-RUN npm ci
+RUN --mount=type=cache,target=/root/.npm npm ci --prefer-offline
 
 FROM base AS builder
 WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
-RUN npm run build
+RUN --mount=type=cache,target=/root/.npm \
+    --mount=type=cache,target=/app/.next/cache \
+    npm run build
 
 FROM base AS runner
 WORKDIR /app