Cannot mount host directory with mattermost Docker container

I'm using the mattermost container 2.0 (https://github.com/mattermost/platform/tree/master/docker/2.0), and I'm trying to mount the /mattermost/data volume to the host directory - but files created in the container are not visible in the host. Here is the result of docker inspect mattermost:

{
    "Source": "/mnt/mattermost-data/data",
    "Destination": "/mattermost/data",
    "Mode": "",
    "RW": true
},
{
    "Source": "/mnt/mattermost-data/db",
    "Destination": "/var/lib/mysql",
    "Mode": "",
    "RW": true
}

I've tried to set the permission to 777 to the folders, but still no luck.
What do I need to do to persist on the host the data, db, and config folders?