Unlike net/http/internal.chunkedReader, this has an interface where we can handle individual chunks.

ErrNoSuchImage replaces docker.NoSuchImage, which does not contain the image name, which in turn breaks docker clients post 1.7.0 since they expect the image name to be present in errors.

MalformedHostHeaderOverride is a wrapper to be able to overcome the 400 Bad request coming from old docker clients that send an invalid Host header.

MalformedHostHeaderOverrideConn wraps the underlying unix connection and keeps track of the first read from http.Server which just reads the headers.