how do I tell the machine to send piefed traffic to this subdomain and joplin traffic (for example) to another domain

You don’t send traffic to domains. You point all the domains to one host, and on that host, set up a reverse proxy like nginx, caddy, or traefik, and then configure HTTP routing rules. That proxy can run in docker. I use traefik and it does all the routing automatically once I add labels to my docker-compose file.