Though the SearchGuard doc says it should be an absolute URL, you can use a relative URL. If you are using Kibana docker and have set, for example, brandimage to /plugins/home/assets/logos/logo.svg. Then the place to put your logo in the container will be at /usr/share/kibana/src/plugins/home/public/assets/logos/logo.svg. In fact, the default open distro logo is located here as well as other company logos.
I believe you can place into any directory under src that has a “public” in its name, but I have not tested that fully.