By default it returns a shell:
tux@ultron:~/github/kubernetes/cluster/images/hyperkube> docker run -it k8s.gcr.io/debian-hyperkube-base-amd64:0.12.0
sh-4.4# 

There are symlinks for: apiserver, kubectl, kubelet, proxy, scheduler

lrwxrwxrwx   1 root root   10 Oct 19 23:24 apiserver -> /hyperkube
drwxr-xr-x   1 root root  912 Oct 19 23:25 bin
drwxr-xr-x   1 root root    0 Jun 26 12:03 boot
lrwxrwxrwx   1 root root   10 Oct 19 23:24 cloud-controller-manager -> /hyperkube
lrwxrwxrwx   1 root root   10 Oct 19 23:24 controller-manager -> /hyperkube
drwxr-xr-x   5 root root  360 Nov 20 15:36 dev
drwxr-xr-x   1 root root 1810 Nov 20 15:36 etc
drwxr-xr-x   1 root root    0 Jun 26 12:03 home
lrwxrwxrwx   1 root root   10 Oct 19 23:24 kubectl -> /hyperkube
lrwxrwxrwx   1 root root   10 Oct 19 23:24 kubelet -> /hyperkube
drwxr-xr-x   1 root root  154 Oct 19 23:25 lib
drwxr-xr-x   1 root root   40 Oct 11 00:00 lib64
drwxr-xr-x   1 root root    0 Oct 11 00:00 media
drwxr-xr-x   1 root root    0 Oct 11 00:00 mnt
drwxr-xr-x   1 root root    6 Oct 19 23:25 opt
dr-xr-xr-x 434 root root    0 Nov 20 15:36 proc
lrwxrwxrwx   1 root root   10 Oct 19 23:24 proxy -> /hyperkube
drwx------   1 root root    0 Oct 18 22:56 root
drwxr-xr-x   1 root root   26 Oct 19 23:25 run
drwxr-xr-x   1 root root 1820 Oct 19 23:25 sbin
lrwxrwxrwx   1 root root   10 Oct 19 23:24 scheduler -> /hyperkube
drwxr-xr-x   1 root root    0 Oct 11 00:00 srv
dr-xr-xr-x  13 root root    0 Nov 20 15:32 sys
drwxrwxrwt   1 root root    0 Oct 19 23:25 tmp
drwxr-xr-x   1 root root   70 Oct 11 00:00 usr
drwxr-xr-x   1 root root   90 Oct 11 00:00 var


The CMD is part is the following:

      "Cmd": [
         "/bin/sh",
         "-c",
         "#(nop) COPY file:31e4ddd5c221b63054724ec24849afee6e4dbc386a9cb7f5c8a778ce4a15d255 in /hyperkube "
      ],

The Dockerfile itself it plain simple:

FROM BASEIMAGE

# Copy the hyperkube binary
COPY hyperkube /hyperkube