Loki docker logging plugin with client certificate authentication

I’m trying to figure out, how to use loki docker logging plugin with client certificate authentication. Should be possible by setting loki-tls-cert-file, but I get all the time:

ERROR: for grafana_service  Cannot start service grafana_service:
failed to initialize logging driver: error creating logger:
error creating loki logger: unable to use specified client cert
(/var/ansible/docker/grafana/test.pem)
& key (/var/ansible/docker/grafana/test.pem):
open /var/ansible/docker/grafana/test.pem: no such file or director

Does anybody got it working with client certificate or knows how to provide files to an docker plugin?