Tweaks to Dockerfile tutorial

Made a few tweaks to Dockerfile tutorial links and removed some cruft from the tutorial itself.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Maybe replace with https://docs.docker.com/articles/dockerfile_best-practices/ in stead of removing entirely?

(hm, seeing that link, why does that contain an underscore?)