Better example for data container

Run sleep in data container so it doesn't default to the CMD which would mean running another unnecessary/potentially problematic instance of the actual application/service.
@SvenDowideit I was under the impression that doing it that way would be replacing the default CMD and the way @cpuguy83 said was setting/replacing the ENTRYPOINT.  I am not sure it matters at all whether it runs bin/sh or runs bin/echo, so I am guessing the slightly simpler way you suggest is better.