Add support for devices with "service create"

Initially reported: https://github.com/docker/docker/issues/24865, but I realized it actually belongs here. Feel free to close the other one if you want. Content of the original issue copied below.

Related: #1030

Currently, it's not possible to add devices with docker service create, there is no equivalent for docker run --device=/dev/foo.

I'm an author of nvidia-docker with @3XX0 and we need to add devices files (the GPUs) and volumes to the starting containers in order to enable GPU apps as services.
See the discussion here: https://github.com/docker/docker/issues/23917#issuecomment-233670078 (summarized below).

We figured out how to add a volume provided by a volume plugin:

$ docker service create --mount type=volume,source=nvidia_driver_367.35,target=/usr/local/nvidia,volume-driver=nvidia-docker [...]

But there is no solution for devices, @cpuguy83 and @justincormack suggested using --mount type=bind. But it doesn't seem to work, it's probably like doing a mknod but without the proper device cgroup whitelisting.

$ docker service create --mount type=bind,source=/dev/nvidiactl,target=/dev/nvidiactl ubuntu:14.04 sh -c 'echo foo > /dev/nvidiactl'
$ docker logs stupefied_kilby.1.2445ld28x6ooo0rjns26ezsfg
sh: 1: cannot create /dev/nvidiactl: Operation not permitted

It's probably equivalent to this:

$ docker run -ti ubuntu:14.04                      
root@76d4bb08b07c:/# mknod -m 666 /dev/nvidiactl c 195 255
root@76d4bb08b07c:/# echo foo > /dev/nvidiactl
bash: /dev/nvidiactl: Operation not permitted

Whereas the following works (invalid arg is normal, but no permission error):

$ docker run -ti --device /dev/nvidiactl ubuntu:14.04
root@ea53a1b96226:/# echo foo > /dev/nvidiactl
bash: echo: write error: Invalid argument

The issue names no files or tests. Start by tracing how docker service create handles mounts and how docker run --device applies device permissions and cgroup whitelisting. Done means a service-created task can expose a device such as /dev/nvidiactl with the required permissions, while preserving the existing mount behavior.