Feature: Non-root dockerized test runner example

The current test runner examples run as the root user. Use methods from the BackEnd containerization to run as a user with limited rights.

assigned to @12mumi1mst

AGID is the (argument) group id for the image upon build time. 137 should read `getent... as in the backend documentation. The rest of your assumptions is correct.

Is this the JDK21 runner from #6 (closed)? If that is running without the change to non-root, please push to a new repo named similarly to the JDK17 runner, comment there and close the ticket. The first error says the system is not logged in to docker hub and the image is not public. Maybe you need to change settings on dockerhub for public availability. The second one you could test looking into the runner. If necessary, build a version pausing before termination, so that the container is not deleted.

changed title from Non-root dockerized test runner example to Feature: Non-root dockerized test runner example