Add own docker registry to rancher-os with os-config.tpl.yml

I was able to change the url for the cloud-init.yml.
Now I like to add my own docker registry so and then pre-load images from there.
And also write those changes on first boot to the /dev/sda like with “ros istall -c http://…”
How do I need to change the os-config.tpl.yml?