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)
Add Put() to graphdriver API and use it

This makes all users of Put() have a corresponding call
to Get() which means we will be able to track whether
any particular ID is in use and if not unmount it.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Remove container.EnsureMounted

This was deprecated already and all it did was call Mount().
The use of this was a bit confusing since we need to pair Mount/Unmount
calls which wasn't obvious with "EnsureMounted".

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devicemapper: Unmount inactive devices

This implements the new Put() operation such that
Get()/Put() maintains a refcount for each ID, mounting
only on first Get() and unmounting on the last Get().

This means we avoid littering the system with lots of mounts
and active devicemapper devices and free resources related
to them.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

asdfljk
aufs: Unmount inactive devices

This implements the new Put() operation such that
Get()/Put() maintains a refcount for each ID, mounting
only on first Get() and unmounting on the last Get().

This means we avoid littering the system with lots of mounts
and free resources related to them.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Add a --signal option to the kill command to specify a signal.

Docker-DCO-1.1-Signed-off-by: Paul Lietar <paul@lietar.net> (github: plietar)
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 #3712 from plietar/kill-signal

Add a -s option to the kill command to specify a signal.
Merge pull request #3073 from alexlarsson/refcount-driver-mounts

Refcount driver mounts
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.