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.

.dockerignore

@@ -0,0 +1,9 @@
+node_modules
+**/node_modules
+dist
+**/dist
+.git
+.env
+.env.example
+*.log
+data