Add support for `RUN [""]` in Dockerfiles for greater specificity in what is actually executed

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
change the documentation only - It looks like we have a number of cmdline params to completely update from the code version

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #3558 from tianon/buildfile-run-json

Add support for RUN [""] in Dockerfiles
Merge pull request #3713 from SvenDowideit/document-bind-mount-file

bind mounts work for files - give an example, as its very very useful.
Merge branch 'master' of https://github.com/dotcloud/docker