Differ is updated to set a label for the uncompressed hash
of compressed content. This allows compressed blobs to
be used and looked up for their uncompressed hashes.
Uses commit options to set labels.
Updates rootfs default to create compressed archives.
Signed-off-by: Derek McGowan <derek@mcgstyle.net>
After some analysis, it was found that Content.Reader was generally
redudant to an io.ReaderAt. This change removes `Content.Reader` in
favor of a `Content.ReaderAt`. In general, `ReaderAt` can perform better
over interfaces with indeterminant latency because it avoids remote
state for reads. Where a reader is required, a helper is provided to
convert it into an `io.SectionReader`.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
