[cloudbuild] support for images

[bradrydzewski]

Google Cloud Build has a setting that pushes images on the machine to their registry
https://cloud.google.com/build/docs/build-config-file-schema#images

The images field in the build config file specifies one or more Linux Docker images to be pushed by Cloud Build to Container Registry. You may have a build that performs tasks without producing any Linux Docker images, but if you build images and don't push them to the registry, the images are discarded on build completion

steps:
- name: 'gcr.io/cloud-builders/docker'
 args: ['build', '-t', 'gcr.io/myproject/myimage', '.']
images: ['gcr.io/myproject/myimage']

In order for this to work for the cloud runtime:

1. mount /var/run/docker.sock into steps
2. add step that performs docker login
3. add step that runs docker push for the specified images

In order for this to work for the kubernetes runtime:

1. mount /var/run/docker.sock as a shared volume
2. create a docker-in-docker background step
3. append a step that executes docker login
4. append a step that executes docker push for the specified images