chunked: flush the input tarball to the output

Flush the entire input tarball to the output in the zstd:chunked
stream writer.  This is needed to include any trailing zeros that
affect the uncompressed digest.

Closes: containers#1771

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/chunked/compressor/compressor.go

@@ -420,6 +420,14 @@ func writeZstdChunkedStream(destFile io.Writer, outMetadata map[string]string, r
 		zstdWriter.Close()
 		return err
 	}
+
+	// make sure the entire tarball is flushed to the output as it might contain
+	// some trailing zeros that affect the checksum.
+	if _, err := io.Copy(zstdWriter, its); err != nil {
+		zstdWriter.Close()
+		return err
+	}
+
 	if err := zstdWriter.Flush(); err != nil {
 		zstdWriter.Close()
 		return err