This uses the default provided retry transport by
go-containerregistry as this originally had no retries
built in.
This is useful to avoid intermittent failures of image
registries when returning a retryable status code.
Add tests. The tests assume a POSIX file system, but it seems many
other tests assume Linux, so perhaps this is not a problem, or at
least does not add a new problem.
Fixes#1235
If the DOCKER_CONFIG environment variable is set, use it when
determining if the Docker config file exists. Fall back to kaniko
default if it the DOCKER_CONFIG environment variable is not set.
Fixes#1228