Support for multi-stage builds in dockerbuilder

[Cryptophobia]

Provide guides and instructions on how to update a cluster's Docker version so that the Dockerbuilder component can use multi-stage builds:

https://docs.docker.com/develop/develop-images/multistage-build/

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In https://github.com/deis/builder/blob/master/pkg/gitreceive/k8s_util.go
"this is the code that sets up the environment to run a deis/dockerbuilder pod. Then it hands off control essentially to a dockerbuilder image, and it calls Docker through a python client from https://github.com/deis/dockerbuilder/blob/master/rootfs/deploy.py

Maybe in that code you could at least detect if the Docker version was >= 17.05 and give a warning or disallow multi-stage builds. Or maybe Docker would already handle that for you."