Delete unused volumes

Filters to process on the prune list, encoded as JSON (a map[string][]string).

Available filters:

• label (label=<key>, label=<key>=<value>, label!=<key>, or label!=<key>=<value>) Prune volumes with (or without, in case label!=... is used) the specified labels.
• all (all=true) - Consider all (local) volumes for pruning and not just anonymous volumes.