The scratch image used to be a regular image, but as of
commit 8936789919c5c8004f346f44a3452d1521818b60 it is a
special case, and cannot be used with 'docker pull.'
Update this doc to reflect the new behavior and clear up
confusion surrounding this image.
Signed-off-by: Christy Perez <christy@linux.vnet.ibm.com>
Upstream-commit: 77bd53adfe6877acd1ff8f07cf148f1dbe0b425d
Component: engine
f298ea7477