Add Docker build + Gitea Actions CI for self-hosting

- next.config: output 'standalone' for a self-contained server bundle
- Dockerfile: multi-stage Next.js build (node:22-alpine)
- .gitea/workflows/deploy.yml: build + push to the Gitea registry

next.config.ts

@@ -1,6 +1,7 @@
 import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
+  output: 'standalone', // self-contained server bundle for the Docker image
   images: {
     remotePatterns: [
       {