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>
Export as a tar (Note: "-" can be used for stdout):
$ ctr images export /tmp/oci-busybox.tar docker.io/library/busybox:latest
Import a tar (Note: "-" can be used for stdin):
$ ctr images import foo/new:latest /tmp/oci-busybox.tar
Note: media types are not converted at the moment: e.g.
application/vnd.docker.image.rootfs.diff.tar.gzip
-> application/vnd.oci.image.layer.v1.tar+gzip
Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
