Customizing startup options on Ubuntu

I’ve just installed docker as per the instructions - I have docker-ce 17.03.1~ce-0~ubuntu-yakkety

I wanted to allow the docker daemon to start automatically & enable tcp connectivity

First I tried creating a /etc/docker/daemon.json as per

However this failed with an error
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

I then tried modifying /etc/default/docker with the line

Use DOCKER_OPTS to modify the daemon startup options.

DOCKER_OPTS="–dns --dns -H tcp://"

However these parms don’t appear to be passed.

What is the correct way to do this on ubuntu (obviously I could just startt the daemon manually/in my own script, but I’m intrigued as to what is intended…)