tests: run docker tests in macOS out of box
tests: run docker tests in macOS out of box

Use `/tmp` as temporary directory for docker driver tests, so tests can
run out of the box without any intervention.

macOS sets tempdir as `/var`, which Docker does not whitelist as a path
that can be bind-mounted.

hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,hashicorp/nomad