`linuxkitprojects/kubernetes` → `linuxkitprojects/kubelet`
`linuxkitprojects/kubernetes-image-cache-*` → `linuxkitprojects/kubernetes-docker-image-cache-*`
`kubernetes` subdirectory is also renamed to `kubelet`.
`image-cache` subdirectory is not renamed since we may want to build other
sorts of image cache at some point.
Signed-off-by: Ian Campbell <ijc@docker.com>
