The Moodle CodeRunner question type plugin requires a Jobe server on which to run student-submitted jobs. This repository provides a basic Jobe server that runs all the standard languages but does not have a mysql server installed so cannot use API-key access. For normal use, that's not a problem - API-key access is relevant only to Jobe servers delivering services to multiple clients.
With thanks to David Bowes who did all the hard work on this.
sudo docker run -p 4000:80 trampgeek/jobeinabox:latest
The Jobe server within the CodeRunner admin settings panel should then be set to <host_running_docker>:4000.
Note though that while the container in which this Jobe runs should be secure, the container's network is currently just bridged across to the host's network. This means that Jobe can be accessed from anywhere that can access the host and can access any URI that the host can access. Firewalling of the host is essential for production use.