Add seekable method for TarReader use to determine whether the curren…

…t reader supports seek

Signed-off-by: zyfjeff <[email protected]>

builder/src/tarball.rs

@@ -85,6 +85,15 @@ impl Read for TarReader {
     }
 }
 
+impl TarReader {
+    fn seekable(&self) -> bool {
+        matches!(
+            self,
+            TarReader::File(_) | TarReader::BufReaderInfoSeekable(_)
+        )
+    }
+}
+
 impl Seek for TarReader {
     fn seek(&mut self, pos: SeekFrom) -> std::io::Result<u64> {
         let seekable = self.seekable();