Add volumes for docker driver

[cbednarski]

Similar in theme to #60 and #61, we want to share data between tasks in a group. We will need to mount the task group allocation directory as a docker volume so we can share data between these processes.

[zrml]

+1
Fundamental!
Thanks

[crhuber]

+1

[sshi100]

+1

[dadgar]

This PR closes this #290

[calvn]

Is the doc updated with this? I don't see anything on https://nomadproject.io/docs/drivers/docker.html regarding volumes.

[diptanu]

@cleung2010 So this issue was about tasks in a group sharing an ephemeral volume. If you're looking for mounting host volumes in docker containers, that would land in the future.

[c4milo]

@diptanu do you mean we cannot mount parts of the host's filesystem into Docker containers right now?

[calvn]

@c4milo I think that's the case, don't see that in the docs or source. I believe @diptanu's response confirms it.

[c4milo]

@cleung2010 I believe you can pass -v arguments to the docker command, but I haven't tested it to be really sure. That's why I'm asking, I was under the impression this was possible today.

[c4milo]

ah nevermind, it is intended for the actual binary run with the container. So, it is not possible.