Consider supporting additional input credentials for build and cache pull

[guineveresaenger]

As a followup to #495, we may want to consider passing in additional registry credentials to the provider to enable a scenario where we are pulling cache from a different registry than the one the Image resource is targeting.

[blampe]

No plans to implement this in this provider, but it is supported in the new Docker Build provider.

Repository here, announcement here, API docs here.