Update: now documenting the better way, as described in issue 14513.
On earlier versions of Ubuntu (14.04 and before), changing the command line options to the Docker daemon (e.g. to allow using an insecure private registry) was just a matter of editing /etc/default/docker and uncommenting the line starting with #DOCKER_OPTS=.
On Ubuntu 15.04, even though that file still exists, editing it does not have any effect, and I found it quite tricky to work out how to change Docker’s command line, so I wrote it up here.
I wanted to use an insecure private docker registry for Docker on Ubuntu 15.04, which uses systemd.
Under systemd, we must create a config file that overrides the default Docker command line by typing sudo systemctl edit docker. In the editor which pops up, type:
[Service] ExecStart= ExecStart=/usr/bin/docker -d -H fd:// --insecure-registry=myreg.example.com:5000
Note: the first “ExecStart=” line is necessary.
The second “ExecStart=” line should contain whatever command line options you want to include.
With this config in place, restart the Docker service:
$ sudo systemctl restart docker
Check everything looks right for the Docker service:
$ systemctl status docker
And confirm the command line arguments have been applied with:
$ ps axwww | grep /usr/bin/docker