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.

.env.example

@@ -0,0 +1,3 @@
+# Copy to .env to override docker-compose.yml defaults
+HOST_PORT=47174
+IMAGE=repo.explewd.com/explewd/vildhjarta:latest