copy/copy: fail copySingleImage cases on EnsureCompressionVariantsExist

EnsureCompressionVariantsExist is only valid when working with a
manifest list.

Signed-off-by: Aditya R <[email protected]>

copy/copy.go

@@ -262,13 +262,19 @@ func Image(ctx context.Context, policyContext *signature.PolicyContext, destRef,
 	}
 
 	if !multiImage {
+		if len(options.EnsureCompressionVariantsExist) > 0 {
+			return nil, fmt.Errorf("EnsureCompressionVariantsExist is not implemented for copySingleImage")
+		}
 		// The simple case: just copy a single image.
 		single, err := c.copySingleImage(ctx, c.unparsedToplevel, nil, copySingleImageOptions{requireCompressionFormatMatch: false})
 		if err != nil {
 			return nil, err
 		}
 		copiedManifest = single.manifest
 	} else if c.options.ImageListSelection == CopySystemImage {
+		if len(options.EnsureCompressionVariantsExist) > 0 {
+			return nil, fmt.Errorf("EnsureCompressionVariantsExist is not implemented for copySingleImage")
+		}
 		// This is a manifest list, and we weren't asked to copy multiple images.  Choose a single image that
 		// matches the current system to copy, and copy it.
 		mfest, manifestType, err := c.unparsedToplevel.Manifest(ctx)