Edits and fixes based on review.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
I think it'd be fair to remove this specific bit.  Just for reference, a lot of these have grown out of bad habits I've seen forming in the Dockerfiles of other people combined with my own experiences in maintaining a lot of different types of Dockerfiles long-term, so it really is all about the long-term maintainability of the Dockerfile.

So this one specifically is mostly about making sure you have the least amount of duplication possible, which helps avoid having the same package listed twice for no reason (or makes it much more obvious when such a thing occurs), and I think we cover that somewhere else, so this might be better as a tack-on to wherever we cover that since it's the same main purpose.