fix: run container on port 80 to match Dokploy domain routing (fixes 502)

Dokploy domains were configured to route to internal port 80, but the
Node standalone server listened on 3000, causing Bad Gateway. Align
the container's PORT/EXPOSE with Dokploy's routing instead.

Dockerfile

@@ -9,9 +9,9 @@ COPY build/ ./
 # so no `npm install` is necessary here.
 
 # Expose the port that server.js listens on
-EXPOSE 3000
+EXPOSE 80
 
-ENV PORT=3000
+ENV PORT=80
 ENV HOSTNAME="0.0.0.0"
 ENV NODE_ENV=production