gke-windows-builder private registry

I'm looking into using Google Cloud Build and gke-windows-builder for building a Windows container image but I'm running into issues when trying to authenticate to a registry thats not a Google Artifact Registry. Please could you advise how to achieve this? Or point me to the right place to ask this question?

I have ran steps before the gke-windows-builder to perform a docker login but then when the container tries to push it is still saying I'm unauthorized.