Tag docker with node version

Is this possible as part of the release process when [updating the node version](https://github.com/GoogleCloudPlatform/nodejs-docker/commit/59437b4f0ed3ac0e5d2d8467235713391b17f75e)?

By tag I mean against the [image](https://console.cloud.google.com/gcr/images/google-appengine/GLOBAL/nodejs?gcrImageListsize=30)
