list dangling volumes (A dangling volume is a volume that is not currently used by any container.)

docker volume ls -f dangling=true

create a named volume from an anonymous one

• docker ps -a to find container identifier
• docker inspect on container to find out the anon volume mount point
• docker volume create <new_volume_name> to create new named volume
• docker stop to stop the container in order to ensure no changes are made to the date while migrating
• docker cp to copy the data from container (using the found mount point) to a temporary location on host
• cp -r / cp to copy data from host to the created named volume
• update the container configurations to use new named volume
• docker restart to restart the container with the new volume mounted

prune

Docs

Remove all unused local volumes. Unused local volumes are those which are not referenced by any containers. By default, it only removes anonymous volumes.

-a: Remove all unused volumes, not just anonymous ones