compare image filesystems directly using mtree

currently, we rely on digests to detect changes between image builds, and because file modification times affect the digests, we have to use the --timestamp argument when invoking podman build

this has the unfortunate side effect that all images in the registry appear to be created/published at the same date/time, as specified by --timestamp, and this is confusing from a ux point of view but also affects the registry cleanup process

instead, compare the image contents directly using the mtree tool, which is now included in podman images: this way we can ignore timestamps and still take into account other changes to file metadata and contents

on the plus side, we can now drop the post-"apt install" file deletions since mtree can be instructed to ignore files and directories when comparing images

From: https://gitlab.torproject.org/tpo/tpa/base-images/-/commit/a508a5e4381d4890fcc2a344e97e589d8dad54c4