With using RELAY_URL=ws://127.0.0.1 in my .env: 

agent-1  | [2024-10-24T19:47:59.458397+00:00] agent.INFO: Client connecting to ws://127.0.0.1 [] []
agent-1  | PHP Fatal error:  Uncaught Amp\Socket\ConnectException: Connection to tcp://127.0.0.1:80 failed after 3 attempts; previous attempts: Connection to tcp://127.0.0.1:80 @ tcp://127.0.0.1:80 refused, Connection to tcp://127.0.0.1:80 @ tcp://127.0.0.1:80 refused in /app/vendor/amphp/socket/src/RetrySocketConnector.php:50
 

 Interesting,  you have them both running as php processes on the same host or are they running in separate processes? In the latter you would need to enter the ip of the relay-container which publishes that port or connect the host port to the container port using the -p parameter of docker