`docker cp` error when container doesn't exist

Fix cp api to return a 404 notfound if container doesn't exist.
Fixes #4119.

Docker-DCO-1.1-Signed-off-by: Fabio Falci <fabiofalci@gmail.com> (github: fabiofalci)
add a little reenforcement to the help for docker build -q

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Avoid temporarily unmounting the container when restarting it

Stopping the container will typicall cause it to unmount, to keep it mounted
over the stop/start cycle we aquire a temporary reference to it during this time.

This helps with https://github.com/dotcloud/docker/issues/4036

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4164 from fabiofalci/cp_error_message

`docker cp` error when container doesn't exist
Support submodules when building from a gh repo

Docker-DCO-1.1-Signed-off-by: Song Gao <song@gao.io> (github: songgao)
Merge pull request #4180 from alexlarsson/fix-restart-race

Avoid temporarily unmounting the container when restarting it
Merge pull request #3463 from songgao/patch-2

Support submodules when building from a gh repo
Merge pull request #4113 from SvenDowideit/build-quiet-help

add a little reenforcement to the help for docker build -q