The buildImage function is not sufficient to build a Docker image with Nix because the Nix database needs to be initialized (otherwise, the Nix database and Nix store diverge).
I think this is why a Nix image has been build on top of Alpine initially. Later, the function buildImageWithNixDb has been added to cover this usecase.
Note there was an attempt to publish a image built with Nix but we never finished the PR 