For each container, try to retrieve information about image layers paths
(from the host filesystem's perspective) and include them in
`ContainerInfo`.

This field is present only for OverlayFS driver. BTRFS doesn not use it.

The procedure for BTRFS is more complicated than for OverlayFS.
`GraphDriver` doesn't contain straightforward information. Instead, it
has to be retrieved with the following steps:

1. Using the given image ID, find an "imagedb entry". It's located in
   `/var/lib/docker/image/btrfs/imagedb/content/sha256/<image_id>`.
2. Get the list of layer checksums from that entry.
3. For each layer, check whether a "layerdb entry" exists. It's located
   in `/var/lib/docker/image/btrfs/layerdb/sha256/<layer_id>`. The
   layerdb directory contains a `cache-id` file.
4. That `cache-id` file contains an ID of a BTRFS subvolume. The
   subvolume can be found in `/var/lib/docker/btrfs/subvolumes/<cache_id>`.

Use libpod's image store and layer store to determine image layer
directories on the host for Podman.