feat(deploy): Next standalone 출력 + Dockerfile (NAS self-host Phase 3)

- next.config: output 'standalone' + outputFileTracingRoot(workspace 중첩 방지)
- Dockerfile(멀티스테이지, NEXT_PUBLIC_* build-arg) + .dockerignore
- maxDuration은 Vercel 운영 보호 위해 유지(self-host에선 무시됨)
- 로컬 빌드 검증: .next/standalone/server.js 루트 생성 확인

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Dockerfile

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+# 쟁승메이드 Next.js — NAS self-host용 standalone 컨테이너
+# 빌드는 로컬에서(NAS Celeron 빌드 금지). NEXT_PUBLIC_* 는 빌드타임 인라인이라 build-arg로 주입.
+FROM node:20-alpine AS deps
+WORKDIR /app
+COPY package.json package-lock.json ./
+RUN npm ci
+
+FROM node:20-alpine AS builder
+WORKDIR /app
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+ARG NEXT_PUBLIC_SUPABASE_URL
+ARG NEXT_PUBLIC_SUPABASE_ANON_KEY
+ENV NEXT_PUBLIC_SUPABASE_URL=$NEXT_PUBLIC_SUPABASE_URL
+ENV NEXT_PUBLIC_SUPABASE_ANON_KEY=$NEXT_PUBLIC_SUPABASE_ANON_KEY
+ENV NEXT_TELEMETRY_DISABLED=1
+RUN npm run build
+
+FROM node:20-alpine AS runner
+WORKDIR /app
+ENV NODE_ENV=production
+ENV NEXT_TELEMETRY_DISABLED=1
+# next.config output:'standalone' 산출물
+COPY --from=builder /app/.next/standalone ./
+COPY --from=builder /app/.next/static ./.next/static
+COPY --from=builder /app/public ./public
+EXPOSE 3000
+CMD ["node", "server.js"]