Sorry I don’t understand why you don’t understand why you would want to backup potentially broken emails in case of e.g. storage failures as mentioned:
This is going to be obsolete with the upcoming 6.3 any way, but still, you’d want to backup files that potentially are important.
Well the docker container uses a custom backup script that does not exactly match the upstream backup script. This backup script (the custom one) does not expect you to have the storage in the container but in the database. That’s the default for Zammad.
If you still want to have them in the containers file system, then you want to backup /opt/zammad/storage/ besides the database.