fix: chmod new layer to 0o644 when creating it

Signed-off-by: zwwhdls <zww@hdls.me>

server/layer.go

@@ -51,6 +51,9 @@ func NewLayer(r io.Reader, mediatype string) (Layer, error) {
 		if err := os.Rename(temp.Name(), blob); err != nil {
 			return Layer{}, err
 		}
+		if err := os.Chmod(blob, 0o644); err != nil {
+			return Layer{}, err
+		}
 	}
 
 	return Layer{