GitLab Runner on OpenShift with self-signed cert

I run an full functional OpenShift/OKD Cluster, my main GitLab instance works very well. Now I wan’t to install the GitLab Runner Operator, but how do I mount a secret/configMap of self-signed cert into the runner so it can connect? see example of yaml

kind: Runner
  name: gitlab-runner
  gitlabUrl: https://gitlab.domain.tld
  buildImage: alpine
  token: gitlab-runner-secret
  tags: openshift
  volume: ??? (what syntax to use here to map /etc/gitlab-runner/certs/xxxx.crt? can't find it anywhere in documentation)