* Add flag to remap registries for any registry mirror
The purpose of this PR is to add an option to remap registries, a kind of generalized `--registry-mirror`.
This is helpful for air-gapped environments and/or when local registry mirrors are available (not limited to docker.io).
This allows user to reference any images without having to change their location.
It also permit to separate infra related configuration (the mirrors) from CI/CD pipeline definition by using an environment variable for example (the reason behind the early return if flag provided but empty).
Therefore you can have a pipeline calling kaniko with `--registry-map=$REGISTRY_MAP` and have the `REGISTRY_MAP` populated via the runner's env by another team, and the absence of env wouldn't trigger a failure, it makes the pipeline env independent.
I've also considered the option of environment variables directly but it doesn't seems to be in kaniko's philosophy.
This makes quite some duplicated code :/ One option to keep the mirror flag and behavior would be to use only one codebase and convert `--registry-mirror=VALUE` to `--registry-map=index.docker.io=VALUE` internally. Suggestions welcome!
* Configure logging config sooner to be able to use it in flag parsing
* Replace registry mirrors by maps logic and use env var
* Add env vars to README.md
* Fix test
1bf529e6d9