feat(deploy): add Docker Compose self-hosted deployment option

Add multi-platform Docker image build (amd64 + arm64) with GitHub Actions
CI that pushes to GHCR on every merge to main. Users can self-host with
a single `docker compose up -d` command.

- Dockerfile: multi-stage build with Next.js standalone output (~150-200MB)
- docker-compose.yml: one-command self-hosted deployment
- .github/workflows/docker.yml: CI workflow with QEMU cross-compilation
- next.config.ts: conditional `output: "standalone"` via BUILD_STANDALONE env
- README (zh/en/ja): restructure deploy section to include Docker option

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,36 @@
+FROM node:22-alpine AS base
+RUN corepack enable && corepack prepare pnpm@9.12.0 --activate
+
+# --- deps: install production + dev dependencies (cached layer) ---
+FROM base AS deps
+WORKDIR /app
+COPY package.json pnpm-lock.yaml ./
+RUN pnpm install --frozen-lockfile
+
+# --- builder: build Next.js standalone output ---
+FROM base AS builder
+WORKDIR /app
+COPY --from=deps /app/node_modules ./node_modules
+COPY . .
+ENV NEXT_TELEMETRY_DISABLED=1
+ENV BUILD_STANDALONE=true
+RUN pnpm build
+
+# --- runner: minimal production image ---
+FROM node:22-alpine AS runner
+WORKDIR /app
+ENV NODE_ENV=production
+ENV NEXT_TELEMETRY_DISABLED=1
+ENV HOSTNAME=0.0.0.0
+ENV PORT=3000
+
+RUN addgroup --system --gid 1001 nodejs && \
+    adduser --system --uid 1001 nextjs
+
+COPY --from=builder --chown=nextjs:nodejs /app/public ./public
+COPY --from=builder --chown=nextjs:nodejs /app/.next/standalone ./
+COPY --from=builder --chown=nextjs:nodejs /app/.next/static ./.next/static
+
+USER nextjs
+EXPOSE 3000
+CMD ["node", "server.js"]