Add btrfs storage-driver to docker daemon config on install

[hartraft]

I used nand-sata-install to copy the filesystem over to system on sata / usb whilst choosing btrfs as the filesystem.

After using softy to install docker, the docker daemon wouldn't start. All that was needed was to add /etc/docker/daemon.json as mentioned in the docker docs.

{
  "storage-driver": "btrfs"
}

If the docker install could take this into account, that would be great. Thanks!