To backup some_volume to /tmp/some_archive.tar.bz2 simply run:
docker run -it -v some_volume:/volume -v /tmp:/backup alpine \
tar -cjf /backup/some_archive.tar.bz2 -C /volume ./
And to restore run:
docker run -it -v some_volume:/volume -v /tmp:/backup alpine \
sh -c "rm -rf /volume/* /volume/..?* /volume/.[!.]* ; tar -C /volume/ -xjf /backup/some_archive.tar.bz2"
From https://loomchild.net/2017/03/26/backup-restore-docker-named-volumes/