Detect if images are available if we don't have to generate a Package

While building, if we aren't doing a clean build, we scan now to see if
images are available and we skip, in case we don't find a metadata
already.

pkg/compiler/backend/simpledocker.go

@@ -119,6 +119,10 @@ func (*SimpleDocker) ImageExists(imagename string) bool {
 	return true
 }
 
+func (*SimpleDocker) ImageAvailable(imagename string) bool {
+	return imageAvailable(imagename)
+}
+
 func (*SimpleDocker) RemoveImage(opts compiler.CompilerBackendOptions) error {
 	name := opts.ImageName
 	buildarg := []string{"rmi", name}