Fix issue #3375 - Return most recent image from the cache

ImageGetCached searches for an image from the cache. Instead of returning the
first image it finds, it should return the most recently created image. When a
build with --no-cache then adds a new image with the same parameters, it is
used instead of the old, existing image.

Docker-DCO-1.0-Signed-off-by: Sjoerd Langkemper <sjoerd@byte.nl> (github: Sjord)

server.go

@@ -1705,16 +1705,19 @@ func (srv *Server) ImageGetCached(imgID string, config *Config) (*Image, error)
 	}
 
 	// Loop on the children of the given image and check the config
+	var match *Image
 	for elem := range imageMap[imgID] {
 		img, err := srv.runtime.graph.Get(elem)
 		if err != nil {
 			return nil, err
 		}
 		if CompareConfig(&img.ContainerConfig, config) {
-			return img, nil
+			if match == nil || match.Created.Before(img.Created) {
+				match = img
+			}
 		}
 	}
-	return nil, nil
+	return match, nil
 }
 
 func (srv *Server) RegisterLinks(container *Container, hostConfig *HostConfig) error {