required parameters for registry.minio.yaml

Added required parameters region and bucket and optional parameters accesskey and secretkey.

According to the documentation at https://docs.docker.com/registry/storage-drivers/s3/ (and my testing) the region and bucket parameters are required, even for minio deployments. As a normal minio installation also has an accesskey and secretkey, I think it would be good to have those parameters in the example, even though those parameters are optional.