You can specify a location for the TLS client certificate/key with the options tls-cert-file
and tls-key-file
in config.toml
(similar to specifying a self-signed CA).
If you add a PEM-encoded cert/key pair to /etc/gitlab-runner/certs/hostname.{cert/key}
they will get picked up and used automatically, again similar to the self-signed CA options.