Those on 16.04 see comment by @thaJeztah https://github.com/docker/docker/issues/3127
It's also possible to use a daemon.json configuration file instead of /etc/default/docker. The /etc/default/docker is only used for systems running upstart, and not for systems running systemd, so is more portable. Also it allows reloading some configuration settings without restarting the daemon;
I'm on Ubuntu 16.04.1, but it shouldn't matter because this is a cross-distro solution.
Just put this json into /etc/docker/daemon.json:
Worked for me and I didn't have to mess with upstart or systemd.