fix bolt tasks with docker_exp transport

[h0tw1r3]

tldr; #257 broke running bolt commands against docker_exp provisioned containers

Behind the scenes, the bolt docker transport uses docker cp to upload and dowload files to the containers.
Unfortunately, we're using --tmpfs /tmp when creating containers, so docker cp is unable to upload or download to that path (or any tmpfs path). Technically it's uploading files to the container filesystem, but...

From docker cp documentation corner cases:

It isn't possible to copy certain system files such as resources under /proc, /sys, /dev, tmpfs, and mounts created by the user in the container. However, you can still copy such files by manually running tar in docker exec.

Ultimately the problem needs to be solved in the bolt docker transport, but that's a much harder problem to solve. For now, not mounting tmpfs fixes this problem.