Explain better the concept of running ELK in Docker

[DinisCruz]

from #59

"From a practical point of view, Docker makes it very easy for you to try and use a wide variety of applications and environments. For example you can start a local (fully working) instance of the ELK Stack (Elastic search + Logstash + Kibana) in Docker, simply by running the sudo Docker pull sebp/elk command (without installing anything on your host machine).
" -- do you assume the reader is already familiar with these concepts? If yes than ok, but if not you might lose them here with the technical jargon. You might want to add a paragraph or two to explain a bit more about the stack. or maybe move this section altogether to later in the book.
"### It all started with physical containers" - this section could move to before Docker and