How to change dockerd -H configuration on Ubuntu 16.04?

I have Docker version 17.03.1-ce running on Ubuntu 16.04, installed by apt. It starts up fine. I want to change the host it responds on. Adding “-H tcp://” in DOCKER_OPTS in /etc/init.d/docker has no effect, and adding daemon.json to /etc/docker with “hosts” causes the following error (from syslog):

unable to configure the Docker daemon with file /etc/docker/daemon.json: the following directives are specified both as a flag and in the configuration file: hosts: (from flag: [fd://], from file: tcp://)

I cannot determine where the -H is being called (it’s not in /etc/init.d/docker, AFAICT) so that I can override it. Clues are appreciated.

Don’t you have systemd on Ubuntu 16?
The Docker configuration file is not in /lib/systemd/system/docker.service and the flag “-H fd://” set in the ExecStart part of the systemd unit file?

Thanks, that’s exactly what I was looking for. systemd has more than a few places to look.