When using docker-compose deployment, the celerybeat container may stumble over a stale pid file

When using the docker-compose.yml from 0.20.1, the celerybeat container may fail to restart after a clean exit, repeating the following log lines:

2019-12-25 17:01:13,519 funkwhale_api.config INFO     Running with no plugins
:0: UserWarning: You do not have a working installation of the service_identity module: 'No module named 'service_identity''.  Please install it from <https://pypi.python.org/pypi/service_identity> and make sure all of its dependencies are satisfied.  Without the service_identity module, Twisted can perform only rudimentary TLS client hostname verification.  Many valid certificate/hostname mappings may be rejected.
ERROR: Pidfile (celerybeat.pid) already exists.
Seems we're already running? (pid: 1)
celery beat v4.3.0 (rhubarb) is starting.

This can be worked around by executing

rm "$(docker inspect funkwhale_celerybeat_1 | jq -r '.[0].GraphDriver.Data.UpperDir')"/app/celerybeat.pid 

after host reboots.