Support internal docker repository by default

[berndtj]

Feature Request

Detailed Description

Rather than depend on an external repository (dockerhub/gcr.io), allow configuring for an internal repository and in fact make that the default (and part of installation).

This makes development and testing far faster and there are bugs with authentication to gcr.io currently within Knative/Kaniko

Context

Possible Implementation

Complexity

• Low - Simple enhancement or bug fix, no architectural changes or refactoring
• Medium - Change requires some thought, but is relatively isolated
• High - Significant architectural change or large refactor