Edits and fixes based on review.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Is there a general change in headings format which I might have missed? (if so sorry about that!)

I believe this should be: Best practices for writing Dockerfiles

Better yet,

Best practices for writing a Dockerfile

And even better, I'd replace `writing` with `crafting` (very personal choice though, so we can just ignore this one ;-)).