It'd be great to have custom domains for preview environments, so that instead of [https://myapp-pr-123.up.railway.app](https://myapp-pr-123.up.railway.app), users can have [https://myapp-pr-123.mydomain.com](https://myapp-pr-123.mydomain.com).

Use cases include eliminating cross-origin concerns, shared cookies, etc. for those environments.