UX: containerd integration: discuss what to include in "container" size

### Description

Relates to https://github.com/moby/moby/pull/45347#discussion_r1168578921

We need to look what makes most sense to account for container "size"; currently we include:

- size of all snapshots used (unpacked layers of the image)
- size of the writable layer


But we _don't_ take the packed (compressed / blobs) in the content store into account.
