systemd unit files that help me
Create a /etc/selenium-hub with the images names you want to use for each container:
e.g.
CONTAINER_HUB="portnumber53/rpi-selenium-grid-hub"
Documentation for Selenium Hub unit files:
http://blog.portnumber53.com/2016/07/05/systemd-a-few-sample-scripts-to-run-selenium-hub/