マウントポイントを調べる

 $ docker inspect -f '{{json .Mounts}}' myapp_db_1 | jq
 [
   {
     "Type": "volume",
     "Name": "6c543b79614428",
     "Source": "/var/lib/docker/volumes/6c543b79614428/_data",
     "Destination": "/var/lib/mysql",
     "Driver": "local",
     "Mode": "",
     "RW": true,
     "Propagation": ""
   }
 ]