In order to do more advanced spec generation with images, snapshots,
etc, we need to inject the context and client into the spec generation
code.
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
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>
