log show umount mnt. I run umount already show not mounted. but  I run kpartx -d /dev/loop0 also show device busy. 
```
2022-04-13 08:42:56.123 | INFO diskimage_builder.block_device.utils [-] Calling [sudo umount /var/cache/nodepool/dib_tmp/dib_build.3DMuQbbI/mnt/]
2022-04-13 08:42:56.675 | INFO diskimage_builder.block_device.utils [-] Calling [sudo kpartx -d /dev/loop0]
2022-04-13 08:42:56.715 | DEBUG diskimage_builder.block_device.utils [-] exec_sudo: device-mapper: remove ioctl on loop0p2  failed: Device or resource busy exec_sudo /usr/lib/python3.6/site-packages/diskimage_builder/block_device/utils.py:135
```