Is serving a civil complaint summons to a door man or concierge of a building considered legal proof of service?

Laws vary by location. Since service is location specific you location will be necessary.

I live in New York City, NY.

Yes, because it is such a person's job to deliver items to residents.

Serving the process on the doorman is one thing. The post by ScottGem suggests that it is proper service in New York City.

"Proof of service" is something else. It is filing a document in court describing how service was achieved. Whether the service so proven is valid is the other issue (discussed above). One would have to review the pertinent court rules and statutes to determine if it is effective service of process.