fix(image): use compatible findmnt mount guard options

hydra/build.py

@@ -60,7 +60,7 @@ sys.exit(code)
 
 def rootfs_retry_guard_script():
     """Refuse deletion of a still-mounted export, including nested mountpoints."""
-    return r"""mounts=$(findmnt --list --raw --noheadings --output TARGET)
+    return r"""mounts=$(findmnt --raw --noheadings --output TARGET)
 while IFS= read -r mountpoint; do
   case "$mountpoint" in
     "$root"|"$root/"*) echo 'Refusing to remove a mounted rootfs export' >&2; exit 1 ;;

tests/test_image.py

@@ -128,6 +128,14 @@ class ImageTests(unittest.TestCase):
                 self.assertEqual(actual, expected)
                 self.assertEqual(result.returncode == 0, not failed and not cleanup_failed)
 
+    def test_retry_guard_uses_valid_real_findmnt_options(self):
+        from hydra.build import rootfs_retry_guard_script
+        self.file('.hydra-export-owned')
+        script = 'set -eu\nroot=' + shlex.quote(str(self.root)) + '\n' + rootfs_retry_guard_script()
+        result = subprocess.run(['bash', '-c', script], capture_output=True, text=True)
+        self.assertEqual(result.returncode, 0, result.stderr)
+        self.assertFalse(self.root.exists())
+
     def test_retry_refuses_removing_owned_rootfs_with_stale_child_mount(self):
         from hydra.build import rootfs_retry_guard_script
         marker = self.file('.hydra-export-owned')