Data duplication in challenge image layers

[zardus]

I suspect that docker's COPY --from=whatever / /, whether --link is used or not, duplicates the base data of shared and/or already-copied container layers. This is the result of building a challenge-granular on #258:

All those 1.5G layers should be empty, but I think they're just duplicating the data of the layers above them. This is a killer issue on the granular PR, but almost certainly hurts us anyways.

[zardus]

The issue: moby/moby#21950