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.
oh wow.

so normally, we write 
```
$ sudo docker run -d -v /dbdata --name dbdata training/postgres echo Data-only container for postgres
```

I'm a fan of mixing things up, but people tell me its not a good idea to do ad-hoc.

@jamtur01 @fredlf