d-f-s: ensure the images are referencing the containers
When we try to remove an image, we may fail if there is a stopped container that references it.
Right now, the code checks only if a container attached to an image is running, but we should also keep a list of all the containers attached to an image.
We should probably
Then, either we:
- prevent any image that still have attached containers to be freed
- or when purging such image, also purge the attached containers
Note that when the image is expired because of the fdo.expired-after
tag, we should likely clean up the attached containers no matter what.