How do I map ports (-p 3001:4000) for a Docker image on Render?

1

I’m creating a Render Web Service from a Docker image, but the deployment keeps failing with this error:

No open ports detected, continuing to scan...
Port scan timeout reached, no open ports detected.
Bind your service to at least one port.

Locally, I run the container like this and it works fine:

docker run -p 3001:4000 my-image

This maps host port 3001 → container port 4000.

However, on Render I don’t see a way to specify -p 3001:4000.
How do I correctly configure port mapping on Render for a Docker-based web service?

  • Which port should my app bind to inside the container?

  • Tried uisng PORT environment variable, that didn’t work

  • Is there any equivalent of -p on Render?