cmd: build minimal docker images for all cmds

Fixes #19 

Build the images using context tar injection (see https://github.com/docker/docker/pull/5715) on top of `FROM scratch`.

This could be cleaner (and work with the hub!) once https://github.com/docker/docker/pull/8021 is merged.
```
$ ./hack/build-docker-images.sh
...
kubernetes/proxy                latest                c8ecf48b90e2        4 days ago          6.073 MB
kubernetes/kubelet              latest                e0aa1bbef6f0        4 days ago          6.946 MB
kubernetes/kubecfg              latest                310aef0bed1c        4 days ago          6.398 MB
kubernetes/integration          latest                d7e55ac70f5d        4 days ago          7.614 MB
kubernetes/controller-manager   latest                0bc6f96a1151        4 days ago          6.094 MB
kubernetes/apiserver            latest                cd8907e1464f        4 days ago          8.945 MB
```

Note: because `scratch` doesn't have `hostname` you have to pass `--hostname_overide` to the `kubelet` when starting.

/cc @gbin
Build targets are already defined in hack/config-go.sh (returned by function kube::default_build_targets). As it is now, you won't compile integration but you will have the scheduler.