Improve warn messages for empty tag scenarios

Distinguish between registry returning no tags vs strategy/constraint
filtering all tags out, so operators can diagnose the root cause.

Signed-off-by: Mark Liu <mark@prove.com.au>

Author: mark-liu

registry-scanner/pkg/image/version.go

@@ -100,7 +100,11 @@ func (img *ContainerImage) GetNewestVersionFromTags(ctx context.Context, vc *Ver
 
 	// It makes no sense to proceed if we have no available tags
 	if len(availableTags) == 0 {
-		logCtx.Warnf("no tags found for image %s in registry", img.GetFullNameWithoutTag())
+		if len(tagList.Tags()) > 0 {
+			logCtx.Warnf("no tags for image %s matched by the %s strategy", img.GetFullNameWithoutTag(), vc.Strategy)
+		} else {
+			logCtx.Warnf("no tags found for image %s in registry", img.GetFullNameWithoutTag())
+		}
 		return nil, nil
 	}
 
@@ -154,6 +158,7 @@ func (img *ContainerImage) GetNewestVersionFromTags(ctx context.Context, vc *Ver
 		return considerTags[len(considerTags)-1], nil
 	}
 
+	logCtx.Warnf("no tags for image %s matched constraint %q", img.GetFullNameWithoutTag(), vc.Constraint)
 	return nil, nil
 }