Fix calculation of relative path for symlink on docker save

Relative paths are now calculated from a base path rather than from the file path, which gets treated like a directory.
Symlinks will now properly point to the file as "../<layer dir>/layer.tar" rather the incorrect "../../<layer dir>/layer.tar".

Fixes #24951

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

image/tarexport/save.go

@@ -307,7 +307,7 @@ func (s *saveSession) saveLayer(id layer.ChainID, legacyImg image.V1Image, creat
 	defer layer.ReleaseAndLog(s.ls, l)
 
 	if oldPath, exists := s.diffIDPaths[l.DiffID()]; exists {
-		relPath, err := filepath.Rel(layerPath, oldPath)
+		relPath, err := filepath.Rel(outDir, oldPath)
 		if err != nil {
 			return distribution.Descriptor{}, err
 		}