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.

Dockerfile

@@ -0,0 +1,11 @@
+FROM node:22-alpine AS build
+WORKDIR /app
+COPY package.json ./
+RUN npm install
+COPY . .
+RUN npm run build
+
+FROM nginx:alpine
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80