Don't call digest.Digester.Hash() on every digeringReader.Read()
Don't call digest.Digester.Hash() on every digeringReader.Read()

digester.Hash() just copies an interface value, but that apparently
involves a heap allocation, and actually shows up fairly high in
profiles.  So do that once per layer instead of once per copied buffer.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image