dockerfile: normalize platform in image config

Base image may use unnormalized platform so if platform
is inherited normalize needs to be called again.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

frontend/dockerfile/dockerfile2llb/convert.go

@@ -815,6 +815,7 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 			target.image.OSFeatures = append([]string{}, platformOpt.targetPlatform.OSFeatures...)
 		}
 	}
+	target.image.Platform = platforms.Normalize(target.image.Platform)
 
 	return target, nil
 }