When using docker-machine on a Mac only the /Users folder is shared with
the VM. Uploads fail since the normal tmpdir is not shared. This change
uses the local packer directory (usually when run in the users home
folders) allowing it to work without setting TMPDIR explicitly. A better
fix would be to use the docker API directly but that would force users
to use docker API version 20+.
- fixes#901, fixes#1752, fixes#2436, fixes#2675, fixes#2697.
Mark Peek
Seth Vargo
Mitchell Hashimoto