Add Docker support with multi-stage build

Enable output: standalone in next.config.ts so Next.js produces a
self-contained server bundle in .next/standalone. Multi-stage Dockerfile
(deps -> builder -> runner) on node:20-alpine keeps the final image
minimal; app runs as a non-root user for security. .dockerignore
excludes node_modules, .next, .env files, and editor tooling.

To build and run:
  docker build -t portfolio .
  docker run -p 3000:3000 portfolio

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

next.config.ts

@@ -1,7 +1,8 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
-  /* config options here */
+  // Produces a self-contained server bundle in .next/standalone — required for Docker
+  output: "standalone",
 };
 
 export default nextConfig;