The db-userauth and db-notes Dockerfiles contain VOLUME /var/lib/mysql, and when we started the containers, we gave --volume options, assigning a host directory for that container directory:

docker run --name db-notes \
  ...
  --volume `pwd`/../notes-data:/var/lib/mysql \
  ...

We can easily see this connects a host directory, so it appears within the container at that location. Simply inspecting the host directory with tools such as ls shows that files are created in that directory corresponding to a MySQL database.

The VOLUME instruction instructs Docker to create a directory outside the container and to map that directory so that it's mounted inside the container on the named path. The VOLUME instruction by itself doesn't control the directory name on the host computer. If no --volume option is given, Docker still arranges for the content of said directory to be kept outside the container. That's useful, and at least the data is available outside the container, but you haven't controlled the location.

If we restart the db-notes container without using the --volume option for /var/lib/mysql, we can inspect the container to  discover where Docker put the volume:

$ docker inspect --format '{{json .Mounts}}' db-notes
[{"Type":"bind",
"Source":"/Users/david/chap10/frontnet/my.cnf","Destination":"/etc/my.cnf",
"Mode":"","RW":true,"Propagation":"rprivate"},{"Type":"volume","Name":"39f9a80b49e3ecdebc7789de7b7dd2366c400ee7fbfedd6e4df18f7e60bad409",
"Source":"/var/lib/docker/volumes/39f9a80b49e3ecdebc7789de7b7dd2366c400ee7fbfedd6e4df18f7e60bad409/_data","Destination":"/var/lib/mysql",
"Driver":"local","Mode":"","RW":true,"Propagation":""}]

That's not exactly a user-friendly pathname, but you can snoop into that directory and see that indeed the MySQL database is stored there. The simplest way to use a user-friendly pathname for a volume is with the --volume options we showed earlier.

Another advantage we have is to easily switch databases. For example, we could test Notes with pre-cooked test databases full of notes written in Swahili (notes-data-swahili), Romanian (notes-data-romanian), German (notes-data-german) and English (notes-data-english). Each test database could be stored in the named directory, and testing against the specific language is as simple as running the notes container with different --volume options. 

In any case, if you restart the notes container with the --volume option, you can inspect the container and see the directory is mounted on the directory you specified:

$ docker inspect --format '{{json .Mounts}}' db-notes 
[{"Type":"bind",
"Source":"/Users/david/chap10/frontnet/my.cnf","Destination":"/etc/my.cnf",
"Mode":"","RW":true,"Propagation":"rprivate"},
{"Type":"bind",
"Source":"/Users/david/chap10/notes-data","Destination":"/var/lib/mysql",
"Mode":"","RW":true,"Propagation":"rprivate"}]

With the --volume options, we have controlled the location of the host directory corresponding to the container directory.

The last thing to note is that controlling the location of such directories makes it easier to make backups and take other administrative actions with that data.