mkdir /.gitlab-runner: permission denied

I'm trying to follow the Helm chart instructions on setting up the GitLab Runner in an on-prem Kubernetes instance (Tanzu). This required copying the runner image into a local repository, but otherwise appeared to proceed as expected. However, the runner panics on starting with:

PANIC: loading system ID file: saving system ID state file: creating directory: mkdir /.gitlab-runner: permission denied

This is true using the 15.7.1 image (which I assume is Ubuntu) and also the alpine3.13-v15.7.1 image. The docs (https://docs.gitlab.com/runner/configuration/advanced-configuration.html) suggest .gitlab-runner directory should be in /home/.gitlab-runner; the deployment descripter suggests that it should be /home/gitlab-runner/.gitlab-runner (this is the mounted volume). However, the mkdir is attempting to create in root (is PWD not being set?). Is this a configuration issue or a an image issue or did I miss something?