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>

.dockerignore

@@ -0,0 +1,26 @@
+node_modules
+.pnpm-store
+.next
+.open-next
+.wrangler
+.vercel
+.turbo
+.claude
+.git
+
+dist
+build
+out
+
+.env
+.env.local
+.env.*.local
+.dev.vars
+
+.DS_Store
+*.log
+npm-debug.log*
+pnpm-debug.log*
+
+repomix-output.xml
+users.md