Is the Mega picked up by the OS (Buster)?
How did you connect to the serial monitor? Directly via Buster ie:
When you start the composer what do you get in the first few lines of the log? this is mine picking up the controller:
[email protected]:~/kegberry# docker-compose logs -f
Attaching to kegberry_pycore_1, kegberry_workers_1, kegberry_kegbot_1, kegberry_kegboard_1,
kegboard_1 | 2021-01-18 09:00:25,624 INFO (kegnet) Connecting to redis at redis://redis:6379/0
kegboard_1 | 2021-01-18 09:00:25,628 INFO (main) Main loop starting.
kegboard_1 | 2021-01-18 09:00:25,629 INFO (main) Device added: /dev/ttyACM0
kegboard_1 | 2021-01-18 09:00:27,053 INFO (main) unknown device: <HelloMessage:
firmware_version=18 protocol_version=None serial_number= uptime_millis=0 uptime_days=0>
kegboard_1 | 2021-01-18 09:00:27,053 INFO (main) Device <Kegboard path=/dev/ttyACM0
speed=115200> is named: kegboard