The client preview lives at their domain and dies on launch day

The client wants to show the new site to their board before launch, and they want it at preview.theirbrand.com, not at a hostname with 48 hex characters in it.

You create a proxy alias for the container's port 3000 with expires_at set to launch day. They add one CNAME from preview.theirbrand.com to the alias. Within ten minutes the proxy has noticed the record, provisioned a Let's Encrypt certificate, and is serving their site over HTTPS at their name. You didn't touch a cert, a load balancer, or a DNS zone you don't own.

On launch day the alias stops routing on its own. The board's bookmark 404s. The production alias, on the same container, keeps going.