@riverful You are not required to take a backup of the full qemu image, if that is not your requirement. I suspect some people will be interested in backing-up specific volumes or specific files and directories inside a volume. For these people, a full qemu-file backup is too much, they need more granularity and control over what will be backed-up.
On the topic:
My understanding is that, at the moment, it is a matter of getting onto the Docker host and backing up the folders. Make sense and it is simple enough.
For instance, the local docker volume ls command and the ls command on the Docker host should give you the same result (afaik):
$ docker volume ls
DRIVER VOLUME NAME
local 04576d248c19b1210d47e94c8211493428cd3c3aa71dfe3fa0f4214589a6f875
local 31af0f01492d8f7b832dad75e731b754302e84fbecfa7c654d7de10465bec204
local 8c511e67b2859e33493798eacf5db901ca40f6dd43725c5472c0c931fb408442
etc.
$ screen ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/tty
#user: root
#password: <none>
$ ls -ltrh /var/lib/docker/volumes
total 148
drwxr-xr-x 3 root root 4096 May 16 13:20 04576d248c19b1210d47e94c8211493428cd3c3aa71dfe3fa0f4214589a6f875
drwxr-xr-x 3 root root 4096 May 16 13:20 31af0f01492d8f7b832dad75e731b754302e84fbecfa7c654d7de10465bec204
drwxr-xr-x 3 root root 4096 May 16 13:20 8c511e67b2859e33493798eacf5db901ca40f6dd43725c5472c0c931fb408442
etc.
So on the Docker host, I can imagine a backup agent running (or a backup agent connecting to that host) and taking a standard file-based backup of the volumes you’d like to back-up.
Good luck!