Add Docker deployment, matching Djupet's setup

Multi-stage build (node build -> nginx:alpine serve), docker-compose
with a configurable HOST_PORT (default 47174, an unusual port left
free for nginx proxy manager to front later), and a Gitea Actions
workflow that builds/pushes the image on push to main.

Verified: image builds, container serves the app and all art assets
on port 47174.

nginx.conf

@@ -0,0 +1,10 @@
+server {
+    listen 80;
+    root /usr/share/nginx/html;
+    index index.html;
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+    gzip on;
+    gzip_types text/css application/javascript application/json image/svg+xml;
+}