diagram and docs showing an example system composition

This merge request includes a diagram I drew to help my coworkers visualize how the Docker images compose together to make a final system. There are of course many other ways to do this, but I thought including this as an example might be helpful.

Is this of interest? If so, please let me know if you'd like updates or changes that would be helpful. The text is a little rough and I can spend more time on it if there is interest.

Thanks @plajjan for your excellent talks at NSO Days -- they were very helpful for me and my team!!