WarmImage doesn't work with distroless containers

[mattmoor]

Right now the prototype assumes /bin/sh is available for convenience. My assumption was always that I'd replace sleep with pause when I dealt with this, but gcr.io/google-containers/pause doesn't have what we'd need to copy pause to a volume to mount into the other containers.

We should simply publish a tiny go_image from this repo that we can run as an init container to populate the volume (with itself), and then override the entrypoint of the "warm" containers to execute this.