Add gcloud image

[bchrobot]

We need to support pushing to Google Container Registry. Build execution time is currently wasted installing the gcloud bin on every run. Pull that up into a custom Dockerfile:

echo "deb http://packages.cloud.google.com/apt cloud-sdk main" | sudo tee -a /etc/apt/sources.list.d/google-cloud-sdk.list
curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | sudo apt-key add -
sudo apt-get update && sudo apt-get install google-cloud-sdk

gcloud --quiet auth configure-docker