fix missing layers when exporting a full repository

Therer is a bug in the 'skip' decision when exporting a repository
(`docker save repo`)

Only the layers of the first image are included in the archive (the
layers of the next images are missing)

Signed-off-by: Anthony Baire <Anthony.Baire@irisa.fr>
Merge pull request #9009 from a-ba/fix-export-repo

fix missing layers when exporting a full repository