I was able to reproduce this. Logging in to the DigitalOcean instance and running journalctl -xe provides more information:
-- Unit docker.service has begun starting up.
Jul 07 20:03:52 docker-sandbox docker[4930]: `docker daemon` is not supported on Linux. Please run `do
Jul 07 20:03:52 docker-sandbox systemd[1]: docker.service: Main process exited, code=exited, status=1/
Jul 07 20:03:52 docker-sandbox systemd[1]: Failed to start Docker Application Container Engine.
-- Subject: Unit docker.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
Turns out this is a bug already reported on GitHub and fixed in version 0.12.1
You can grab the latest release at: https://github.com/docker/machine/releases/tag/v0.12.1