Run client app in Docker container instead of host machine

There is value of having the client (KStreams app or more basic producer/consumer) run in a Docker image, not just on a user’s host machine. The benefits are:

1. show distributed environment, i.e. bootstrap server is broker:9092` instead of `localhost:9092`
2. Fewer required dependencies on a user’s machine (e.g. gradle, java)

Outcome is that 100% of the tutorial is Docker-based.