How to specify volumes under runners.kubernetes.volumes section for the gitlab-runner

I can see the config.toml for the gitlab-runner instance with kubernetes executor created like this:

...
  [runners.kubernetes]
    host = ""
    bearer_token_overwrite_allowed = false
    image = "ubuntu:16.04"
    namespace = "gitlab"
    namespace_overwrite_allowed = ""
    privileged = true
    service_account_overwrite_allowed = ""
    [runners.kubernetes.volumes]

How and where do we add the config for the [runners.kubernetes.volumes] section? I need to mount a pvc claim.

Thanks