Docker volume data gets stored twice?

Question

I am using a mysql docker image through docker-compose and I get its data stored twice (on the host and in docker data folder). What I want is to have one copy of mysql data stored on the "host". I've read through the docker and docker-compose docs and I don't seem to get how this is supposed to be working.

Here is the docker-compose config I am using:

version: '3.7'

services:
  mysql:
    image: mysql:5.7
    container_name: "mysql"
    ports:
      - "127.0.0.1:3306:3306"
    volumes:
      - mysql-data:/var/lib/mysql
volumes:
  mysql-data:
    driver: local
    driver_opts:
      device: /local/path
      o: bind

When I inspect the disk contents I can see that the mysql data is stored twice:

• inside the docker data folder, e.g. /var/lib/docker/volumes/myapp_mysql-data/_data
• on the "host" at /local/path

Is this "normal"? What do people use to have mysql in docker but its data located somewhere else?

Updated: Here is the output of the docker volume inspect myapp_mysql-data:

docker volume inspect myapp_mysql-data
[
    {
        "CreatedAt": "...",
        "Driver": "local",
        "Labels": {
            "com.docker.compose.project": "myapp",
            "com.docker.compose.version": "1.24.0",
            "com.docker.compose.volume": "mysql-data"
        },
        "Mountpoint": "/var/lib/docker/volumes/myapp_mysql-data/_data",
        "Name": "myapp_mysql-data",
        "Options": {
            "device": "/local/path",
            "o": "bind"
        },
        "Scope": "local"
    }
]

Answer 1

OK, looks like it works as expected, i.e. docker has volume's _data folder binded to the same location as local path. Thanks @danblack!

The reason for my confusion was ncdu tool that I used to see what takes space on disks, which (after I moved the docker data folder to another disk) showed both folders as if they take the same amount of space.