This makes it easy for share functionality across tools to prevent
pushing layers that are not supposed to be re-distributed.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Adds a method for setting a custom media type key prefix used by the
fetch handler.
This allows both overwriting a built-in prefix (for reasons?) as well as
supplying a custom media type.
I added this because I was getting an error on `FetchHandler` when
pulling docker plugin images which have their own media type.
Signed-off-by: Brian Goff <cpuguy83@gmail.com>
