Je réponds à ma propre question :
"Mounts": [
{
"Source": "/home/steph/data-seedbox/steph/data",
"Destination": "/downloads",
"Mode": "rw",
"RW": true,
"Propagation": "rprivate"
},
{
"Source": "/home/steph/data-seedbox/steph/config/medusa",
"Destination": "/config",
"Mode": "rw",
"RW": true,
"Propagation": "rprivate"
},
{
"Name": "f96bddc8e5521a32a3518c7ccd162236b701c001684e0c36334ae64f550d326a",
"Source": "/var/lib/docker/volumes/f96bddc8e5521a32a3518c7ccd162236b701c001684e0c36334ae64f550d326a/_data",
"Destination": "/config/Medusa",
"Driver": "local",
"Mode": "",
"RW": true,
"Propagation": ""
}
],
Il faut un mount sur config/Medusa (et c'était sur la doc...)