Web terminal with docker runner

I’m trying to debug why a test is failing in CI but works locally, so I would like to use the web terminal with the docker gitlab-runner to see the state of the container. I would expect to see the blue debug button in the job side panel, but it is not there. Is there anything I need to do to activate it.

This is on a self-managed instance of the open source version running 13.8.0.