libcontainerd: change the digest used when restoring

For current implementation of Checkpoint Restore (C/R) in docker, it
will write the checkpoint to content store. However, when restoring
libcontainerd uses .Digest().Encoded(), which will remove the info
of alg, leading to error.

Signed-off-by: huang-jl <1046678590@qq.com>
(cherry picked from commit da643c0b8a)
Signed-off-by: huang-jl <1046678590@qq.com>

libcontainerd/remote/client.go

@@ -723,7 +723,7 @@ func (c *client) writeContent(ctx context.Context, mediaType, ref string, r io.R
 	}
 	return &types.Descriptor{
 		MediaType: mediaType,
-		Digest:    writer.Digest().Encoded(),
+		Digest:    writer.Digest().String(),
 		Size:      size,
 	}, nil
 }