You can pass TCP socket address as an upstream for this proxy.
$ git clone https://github.com/baleyko/tracery.git && cd tracery && docker-compose up
either
$ docker run -it --rm --env UPSTREAM=google.com:80 -p 8080:80 baleyko/tracery:latest
and try
curl http://YOUR_DOCKER_MACHINE_IP:8080</code>