docker: Make GCR the default registry (#920)
docker: Make GCR the default registry (#920)

@duftler PTAL
stitchfix/spinnaker,jtk54/spinnaker,ewiseblatt/spinnaker,jtk54/spinnaker,skim1420/spinnaker,ewiseblatt/spinnaker,stitchfix/spinnaker,duftler/spinnaker,skim1420/spinnaker,Roshan2017/spinnaker,ewiseblatt/spinnaker,Roshan2017/spinnaker,skim1420/spinnaker,duftler/spinnaker,duftler/spinnaker,spinnaker/spinnaker,duftler/spinnaker,Roshan2017/spinnaker,imosquera/spinnaker,imosquera/spinnaker,stitchfix/spinnaker,ewiseblatt/spinnaker,spinnaker/spinnaker,imosquera/spinnaker,jtk54/spinnaker,spinnaker/spinnaker,skim1420/spinnaker,spinnaker/spinnaker