pass image label to snapshot

[wangforthinker]

When a image unpacks, invoker could set labels to images, and pass labels to snapshot which is prepared for image. In snapshotter method such as Prepare and Commit, we can differ image and container snapshot by labels.

[allencloud]

Is this related to pouchd's pull request AliyunContainerService/pouch#2525 ? @wangforthinker

[wangforthinker]

@fuweid

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