To do so, have (skopeo copy) work with a types.Image, and replace uses
of types.ImageSource with types.Image where possible to allow the
caching in types.Image to work.
This is a slight behavior change:
- The manifest is now processed through fixManifestLayers
- Duplicate layers (created e.g. when a non-filesystem-altering command is used
in a Dockerfile) are only copied once.
This does not really go into why duplicate layers can happen or why it
is worth supporting that; the code originates from
504e67b867 ,
which does not explain either.
