fix: look up mount point by directory content, not by device path

Agent-Logs-Url: https://github.com/MaxG87/ButterBackup/sessions/4b40e0ca-a4f5-4bc1-a0be-678d60d13fbf

Co-authored-by: MaxG87 <5477952+MaxG87@users.noreply.github.com>

Author: Copilot

tests/test_cli.py

@@ -451,7 +451,15 @@ def test_unmount_error_does_not_cause_content_deletion(
     # It is assumed that the device is still mounted, since the unmounting is mocked to
     # fail.
     mounts = sdm.get_mounted_devices()
-    mount_of_device = next(iter(mounts[str(config.map_name())]))
+    # In some environments (e.g. Docker) the device mapper node appears as /dev/dm-X in
+    # the mount table rather than /dev/mapper/{UUID}. Search all mount points for the
+    # one containing BackupRepositoryFolder instead of looking up by device path.
+    mount_of_device = next(
+        mount_point
+        for mount_points in mounts.values()
+        for mount_point in mount_points
+        if (mount_point / config.BackupRepositoryFolder).exists()
+    )
     expected_backup_repository = mount_of_device / config.BackupRepositoryFolder
     assert expected_backup_repository.exists()
     assert expected_backup_repository.is_dir()