Allow configuring build-time container pull credentials for image building #68
Description
Currently we are only fetching Dockerhub / docker.io credentials during build times, as they are the only registry that has problematic rate limits for public pulls. If we want to support pulling private images during container builds we need to figure out a way to load in other credentials -- either from the repo config or from the system's docker/podman credential store.
originally brought up in #53 (comment)