Added system images

[gbraad-admin]

All containers can be started as a system image by using their respective init process, which in most cases is systemd except for Alpine.

This is great for 'long-running' environments that aren't really ephemeral. The lifecycle of the container can be maintained by the host using a systemd service unit:

$ (cd $HOME/.config/systemd/user && podman generate systemd --name --files devsys)
$ systemctl --user enable --now container-devsys
$ loginctl enable-linger $USER

and can be logged into as devroot and devuser.