Issue with the "Builders and Fly.io" doc

[iloveitaly]

I found an issue with this document.

Title: Builders and Fly.io
Location: https://fly.io/docs/reference/builders/
Source: https://github.com/superfly/docs/blob/main/reference/builders.html.md

Describe the issue

It's not clear how to specify nixpacks as the builder via config instead of passing it via CLI args each deploy.

I also can't seem to get nixpacks to work. I get the following error:

Run:
  docker run -it registry.fly.io/cosma-playground:deployment-01JCRBJ3RP37Q5XG54DED712AJ
The push refers to repository [registry.fly.io/cosma-playground]
Error: failed to fetch an image or build from source: error rendering push status stream: An image does not exist locally with the tag: registry.fly.io/cosma-playground

Addtional info