feat: Improve support for unprivileged hosts (including LXC) (#479)

* * Add fakeroot to extract the dsm system without elevated permissions
* Remove obsolete docker variable "DEV" used to exclude extraction of device nodes

* feat: Detect unprivileged container

* fix: Use fakeroot for mke2fs

---------

Co-authored-by: Kroese <kroese@users.noreply.github.com>