Speakers do not disappear from Airplay destinations when stopping container

[jonathandann]

To Repro

1. docker-compose down to stop the container gracefully OR
2. ^C stops the container when attached with docker-compose up (i.e. without the -d flag).

After stopping the container , the speakers are still listed as airplay destinations on iOS.

Running without docker as a systemd service (i.e., the regular way to run shairport) service shairport-sync stop will stop the service, and the speakers no longer appear as destinations.

To Investigate

1. Is there a docker hook so we can run the same thing that service shairport-sync stop or systemctl stop shairport-sync does?
2. What do those commands currently do?