Load balance SSL backend server with client authentication

I am using Interlock 1.13 with Docker 1.11 and Swarm 1.2.5

I am trying to load balance tomcat server which serve as ESB in my application.

They listen on port 8443, and require HTTPS with client certificate authentication.

Now, I need to inject client certificate which would be used in proxy section of Nginx

backend {
server some-ip:8443;
}

server {
listen 80;
location / {
proxy_ssl_certificate certs/client.crt;
proxy_ssl_certificate_key certs/client.key;

    proxy_pass https://backend;
}

}
I studied the Interlock configuration section, but couldn’t find options which could do this.

I am already using following options

interlock.hostname=test
interlock.domain=local
interlock.ssl=true
interlock.ssl_only=true
interlock.ssl_cert=test.local.crt
interlock.ssl_cert_key=test.local.key
interlock.ssl_backend=true
But couldn’t find options for proxy_ssl_cert, and proxy_ssl_cert_key

How can I achieve this, if I want to use Interlock with Nginx?