Support splitting exported archive into component layers

[bfosberry]

Have you thought of adding support for splitting the tar into component layers after export, and re-assembling it for re-load to cut down on storage and allow remote transfer costs to be reduced sharing layers, similarly to how docker pull/push work

[tonistiigi]

I'm not sure I understand. The cache file tar is very small, it doesn't have any layer data in it. You still get the layer data through regular push/pull.