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>

next.config.ts

@@ -1,6 +1,7 @@
 import type { NextConfig } from "next";
 
 const config: NextConfig = {
+  output: process.env.BUILD_STANDALONE === "true" ? "standalone" : undefined,
   reactStrictMode: true,
   typedRoutes: false,
   turbopack: {