The snapcast https://github.com/badaix/snapcast server reads from /tmp/snapcast/fifo and expects a named pipe, not a simple file.
The fifo file can bew mounted via volumes without trouble.
Avahi discovery does not work with a simple bridged docker network. The clients have to be told the correct address of this server.
A workaround is to use the --net host network mode. As this is considered a security risk I would advise against it.