If image pull fails of context canceled, image layer will perform a
rollback operation. When image layer is released, the diff folder of layer
will be removed first, and then the digest folder will be removed.
If the diff folder fails to be removed, such as operation not permitted or
interrupted by others, both the digest folder and diff folder will remain
on the disk, this will cause image not be complete and not repairable.
So we should remove the digest folder first for image layers rollback
and ensure image can be re-pulled completely.
