use existing redis via socket

[thrillfall]

I am using the docker-compose setup and have an existing redis (cache for mastodon, nextcloud). How can i connect to the existing redis via unix:///var/run/redis/redis-server.sock? I mapped the socket file as volume which works nicely for my mastodon (also docker-compose) setup.

when i set REDIS_ACTIVITY_HOST=unix:///var/run/redis/redis-server.sock
i get

flower_1| [E ...] Failed to capture events: 'Database is int between 0 and limit - 1, not var/run/redis/redis-server.sock:0/2', trying again in 8 seconds.```

[thrillfall]

i found there is a notation for socket connection:
now i have
REDIS_ACTIVITY_HOST=redis+socket:///var/run/redis/redis-server.sock?virtual_host=1
and i get

celery_worker_1  |     return dict(transport=scheme, hostname=host,
celery_worker_1  | TypeError: dict() got multiple values for keyword argument 'virtual_host'
bookwyrm_celery_beat_1 exited with code 1```

[jaschaurbach]

This will be solved once #2604 is ready