Add Docker image for self-hosted deployment

- next.config: output "standalone" for a self-contained server bundle
- Dockerfile: multi-stage build that runs as the node user, read-only-rootfs
  friendly (Next cache symlinked to /tmp), with a healthcheck
- README: document deploying to playground.jun-liu.net

Co-Authored-By: Claude Opus 5.5 <[email protected]>

next.config.ts

@@ -1,7 +1,8 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  // Self-contained server bundle for the Docker image (see Dockerfile).
+  output: "standalone",
   cacheComponents: true,
   partialPrefetching: true,
   turbopack: {