The proxy listens on TCP port ${TCPPROXY_LISTEN_PORT}. The target can be given by the environment variables ${TCPPROXY_HOST} and ${TCPPROXY_PORT}.
Usage example:
docker run -p 80:80 -e TCPPROXY_LISTEN_PORT=80 -e TCPPROXY_HOST=webserver -e TCPPROXY_PORT=8080
will redirect all incoming connections on TCP port 80
to host webserver
port 8080