Add zsh completion for system prune --volumes

Signed-off-by: Jean-Pierre Huynh <jean-pierre.huynh@ounet.fr>
(cherry picked from commit 6b256484ae)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

components/cli/contrib/completion/zsh/_docker

@@ -2376,7 +2376,8 @@ __docker_system_subcommand() {
                 $opts_help \
                 "($help -a --all)"{-a,--all}"[Remove all unused data, not just dangling ones]" \
                 "($help)*--filter=[Filter values]:filter:__docker_complete_prune_filters" \
-                "($help -f --force)"{-f,--force}"[Do not prompt for confirmation]" && ret=0
+                "($help -f --force)"{-f,--force}"[Do not prompt for confirmation]" \
+                "($help)--volumes=[Remove all unused volumes]" && ret=0
             ;;
         (help)
             _arguments $(__docker_arguments) ":subcommand:__docker_volume_commands" && ret=0