How to add a custom header to Loki docker driver POST requests?

Hello!

I’m configuring Loki for the first time and trying to figure something out. I’m pointing my Loki docker driver to a URL configured for mutual TLS authentication via an AWS API Gateway

    logging:
      driver: loki
      options:
        loki-url: "https://mysite.com/loki/api/v1/push"
        loki-tls-cert-file: /etc/docker/client_device.pem
        loki-tls-key-file: /etc/docker/client_device.key

This works and I see data pass through my API gateway into Loki/Grafana just fine (with some guidance from this blog post).

However, our API gateway also runs a Lambda auth to check a certificate revocation list to ensure the cert is still valid and to cache the response. In order to do this, I need to add a custom header like

        headers['x-auth-cache-id'] = EXAMPLE_ID

Obviously I’d rather not fork and have to maintain the docker driver just for a single custom header. Is there some configuration I’m missing that I could use to add a header to my requests?