add trivial HasImage instance for Image

src/Propellor/Property/Docker.hs

@@ -1,4 +1,4 @@
-{-# LANGUAGE FlexibleContexts #-}
+{-# LANGUAGE FlexibleContexts, TypeSynonymInstances, FlexibleInstances #-}
 
 -- | Docker support for propellor
 --
@@ -85,6 +85,9 @@ data Container = Container Image Host
 class HasImage a where
 	getImageName :: a -> Image
 
+instance HasImage Image where
+	getImageName = id
+
 instance HasImage Container where
 	getImageName (Container i _) = i