Added Backup and restore all volumes in README.md

[delfer]

No description provided.

[Guiorgy]

Running for vl in $(docker volume ls | awk '{print $2 }'); do echo ${vl}.tar.gz ; done prints:

VOLUME.tar.gz
... (volumes)
portainer_data.tar.gz

Where the VOLUME.tar.gz is the result of the headers/title (DRIVER VOLUME NAME) on the first line of output. This should be handled.

Running docker volume ls --help we read -q, --quiet Only display volume names, so this option should be added to your command. In fact, running with this option only prints the names (as the description states) so awk can also be removed:

:~ $ for vl in $(docker volume ls -q); do echo ${vl}.tar.gz ; done
... (volume names)
portainer_data.tar.gz