First pass at replacing parent-child metaphor to describe docker links.

Addresses issue #7141

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
"Context", in the sense that I understand it, does not work that way. We are indeed talking about two particular containers here, which we are using as an example of the general case. It's well beyond the purview of github comments, but for revision, I use an applied method based on fancy-pants post-structuralist ideas about reader responses called "Reader Expectation Theory". I was trained by the guy who wrote this book: http://www.amazon.com/Sense-Structure-Writing-Readers-Perspective/dp/0205296327/ref=sr_1_1?s=books&ie=UTF8&qid=1406571824&sr=1-1&keywords=george+gopen. (I urge you to get yourself a copy; it will help your writing.)  It's a method that has worked very, very well for me for a long time now. That method has specific ideas about context and interpretation as related to the structure of prose. And it is that method that is guiding me to include the article "the" here. 

This may well be the most long-winded defense of a single definite article in history. :-)