Merge pull request #8225 from AkihiroSuda/comment-image-created-at

metadata: add comments about `Image.CreatedAt`

metadata/images.go

@@ -145,6 +145,10 @@ func (s *imageStore) Create(ctx context.Context, image images.Image) (images.Ima
 			return fmt.Errorf("image %q: %w", image.Name, errdefs.ErrAlreadyExists)
 		}
 
+		// The value of `image.CreatedAt` passed from the caller is discarded here.
+		// Ideally we should return an error when the value is already set.
+		// However, as `image.CreatedAt` is defined as a non-pointer `time.Time`, we can't compare it to nil.
+		// And we can't compare it to `time.Time{}` either, as `time.Time{}` is a proper timestamp (1970-01-01 00:00:00).
 		if tm := epoch.FromContext(ctx); tm != nil {
 			image.CreatedAt = tm.UTC()
 		} else {