[FEATURE] Docker container autodiscovery 🐳

**The Problem**
Currently, users have to manually type `--listen 80 --to localhost:3000`. If they use Docker, it would be incredible if Relay could just *know* where to route traffic.

**The Solution**
If a user runs `relay --docker`, we should hook into the Docker Engine API (via `/var/run/docker.sock`), read the running containers, and automatically route `http://.localhost` to the container's exposed port.

**Where to start**
Look at how Traefik handles Docker provider autodiscovery. We just need to parse JSON from the Docker unix socket!

Start by examining the relay --docker entry point and the existing routing setup, then review Traefik's Docker provider behavior. Connect to /var/run/docker.sock, inspect the running-container JSON, and determine how exposed ports map to container names. Done means relay --docker routes http://.localhost to each container's exposed port.

Written by the indexing model from the issue text.