Build Kubernetes in Docker

Scripts and Dockerfile to build a container image, build binaries, run tests, etc.  Also copy output back out to the host machine.

The full flow isn't there yet, but this should work for building the basic binaries.  More to follow for building releases.
Inspired by the Docker Dockerfile.  Regardless -- linux/amd64 is special as that is the combo that we'll be supporting for cluster binaries.  The others are just for the client code.