The previous makefile assumed that PWD was populated. This doesn't
happen in certain cases (e.g., when running as a systemd unit). Make
sure we have the PWD variable set before running any target.

This commit adds:

- Systemd sysusers hook is ran on the dockerfile
- The dockerfile also populates the archlinux keyring
- Remove the libalpm hooks as they are uneccessary now
- Moved the package cleanup before the tar step on the makefile

The old makefile required commands to be run as a superuser, which
may become problematic when deploying on the archlinux infrastructure.
Use fakeroot and regular pacman commands to allow for to bootstrap a
rootfs.

Use the most minimal package set