Add a way to remap registries to use pull-though cache

[babs]

Actual behavior

Currently, a pull-through cache mechanism has been implemented to circumvent docker.io rate limiting.

Expected behavior

I would be nice to generalize this option to any registry, being able to remap any registry (gcr, quay etc) to an internal one.
This would be useful for air-gapped installation, make build more reliable, faster, not dependent of external resources, limit transit costs etc or cases when prefilling the cache is not an option.

Additional Information

There is a PR open to address this subject, feedback is welcome (PR #2935).

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko	- N/A
Please check if this error is seen when you use --cache flag	- N/A
Please check if your dockerfile is a multistage dockerfile	- N/A

[babs]

Merged ;)