use docker to run the example

[nromanetti]

Hi Gwen,

It wasn't easy to run your example (the kafka streams maven dependency could not be found), your uber jar is not runnable, plus I did not want to install Kafka on my mac.

So I ended up using docker for the steps described in the README.md In the end, despite the length of the command lines, I find it much more easier to run.

This PR addresses these points.

Thanks for your great work and great book. N.