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.

docker-compose.yml

@@ -0,0 +1,6 @@
+services:
+  app:
+    image: ${IMAGE:-vildhjarta:local}
+    restart: unless-stopped
+    ports:
+      - "${HOST_PORT:-47174}:80"