The number of containers referencing this volume. This field is set to -1 if the reference-count is not available.
Amount of disk space used by the volume (in bytes). This information is only available for volumes created with the \"local\" volume driver. For volumes created with other volume drivers, this field is set to -1 ("not available")
Usage details about the volume. This information is used by the
GET /system/dfendpoint, and omitted in other endpoints. VolumeUsageData