Verifying with busybox has it's advantages but its inconsistent on two grounds:

1. Rest of the docs mention `docker run` to verify installations.

2. Why use busybox to verify Ubuntu? I think Ubuntu on Ubuntu would be clearer 

3. A lot of people won't haven't hears of busybox and it might be confusing.

So I suggests:

a. docker info; or,
b. run .. ubuntu