With this change, you can add --google_json_key=/path/to/key.json to the DAEMON_ARGS of the kubelet, e.g.
nano /etc/default/kubelet
... # Add the flag
service kubelet restart
With this setting, minions will be able to authenticate with gcr.io repositories nearly as smoothly as if K8s were running on GCE.
NOTE: This private key can be used to access most project resources, consider dropping the service account created through this flow to a project READER, or restricting its access to just the GCS bucket containing the container images.
Matt Moore