Previously, the `InspectImage` method of the Docker interface expected a
"pullable" image ref (name, tag, or manifest digest).  If you tried to
inspect an image by its ID (config digest), the inspect would fail to
validate the image against the input identifier.  This commit changes
the original method to be named `InspectImageByRef`, and introduces a
new method called `InspectImageByID` which validates that the input
identifier was an image ID.