Changing the data directory

By default on Fedora, the directory Docker stores data in (images, etc.) is /var/lib/docker. Rather than changing the systemd service file, you can simply create /etc/docker/daemon.json:

    "graph": "/mnt",
    "storage-driver": "overlay2"
overlay2 should be replaced with whatever is shown in docker info.
/mnt should point to the root directory. For example, if you specify /mnt/docker, the final path is /mnt/docker/docker.

After that, reload the changes (it may be a good idea to delete or backup the old directory):

systemctl daemon-reload
systemctl restart docker

After that, you can view docker info to verify your changes.

Credit goes to Piotr Król’s StackOverflow answer.