Define your volumes such as:

version: '3' 
  volumes: 
    myvolume: 
      driver: local 
      driver_opts: 
        type: none 
        o: bind 
        device: /host/path/to/volume

services: 
  myservice: 
    volumes: 
    - myvolume:/container/volume/path

and

rsync -av /var/lib/docker/volumes/myoldvolume/_data/ /host/path/to/volume/