Because Docker will not delete volumes automatically, you will have to delete them manually using the following command:
$ docker volume rm [volume name]
Note: the volume must not be in use in order to remove it!
To view volume details, run
$ docker volume inspect [volume name]
If you inspect a container with
$ docker container inspect [container name] you will find any mounted volumes under "Mounts"
To quickly remove all unused volumes, run
$ docker volume prune and confirm.