Add vim headers to Dockerfile

Sometimes I have to use nano to get the tabs right because it is different than my local
settings. Remove the tab pain from some users. @tianon don't kill me, wanted to see if 
I could get away with this. If so, I could do them all... (shell scripts)? Or should we just forget it?

expandtab replaces tabs with spaces
> On Dec 23, 2014, at 12:20 PM, Jessie Frazelle <notifications@github.com> wrote:
> 
> In Dockerfile <https://github.com/docker/docker/pull/9794#discussion-diff-22232842>:
> 
> > @@ -1,3 +1,5 @@
> > +# vim: set expandtab!:
> > +# vim: noai:ts=8:sw=8
> ya it worked with noexpandtab instead weird
> vim hates me today
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/9794/files#r22232842>.
>