Avoid registry fallback

[fgiannetti]

Hi all!
I want to use a private registry in Kaniko, but only that registry.

I am implementing an image whitelist in the private registry, but when i return a 403 for a non whitelisted image, Kaniko fallback to dockerhub and the image is pulled from there anyway.
Is there a way to prevent this behavior?

Im using the --registry-mirror argument with the private registry address

Thanks very much in advance!