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)
Huh. Well I learned something today. Pretty classic engineer vs. writer perspective. I think of the marks semantically, like a writer; that is, hyphens *connect* words together and spaces (as represented by underscores) are used to separate words (which is why I don't see that filename as inconsistent). Engineers (like the Google engineer in the first link), see hyphens as separators and underscores as, essentially, null. Very interesting (to someone like me, YMMV). I reckon I'll avoid underscores in URL's in the future, even though it chaps my hide to have engineers define interpretive behavior! :-)