Deploy of private service stuck

I am deploying a private nodejs service. The deploy appears to complete successfully but the status remains as “deploy started”.

… and it just failed, I guess because of some render build timeout. Any insights here? How does Render determine the health of a private service for deploy purposes?

Ok I discovered that to deploy successfully, a private service needs to listen on a port. I added a hello-world express app to go along with the rest of my service and now the deploy moves beyond started to live