[Error] overridden systemd units in /etc/systemd/system can cause startup failure

[brimston3]

Describe the bug
Containers with systemctl fail to start if an override systemd unit file exists in /etc/systemd/system that is not zero-length for any unit file that is typically masked by distrobox.

To Reproduce

distrobox create -i docker.io/library/archlinux --init -n vm-arch
distrobox enter vm-arch
sudo systemctl unmask dbus.service
sudo cp /usr/lib/systemd/system/dbus.service /etc/systemd/system/
sudo shutdown
distrobox enter vm-arch

At this point the container will not start and cannot be entered to fix.

Expected behavior
Container starts and gets to a prompt. The pre-existing service unit in /etc/systemd/system/unit.service is not masked or overwritten.

Logs
distrobox-enter-sysd-mask.log
podman-logs-sysd-mask.log

Desktop (please complete the following information):

• Are you using podman or docker? podman
• Which version or podman or docker? podman 4.2.1
• Which version of distrobox? 1.4.2.1

Additional context

[brimston3]

It's probably safe to ignore errors when performing systemctl mask "$(basename "${unit}")". The other option is to check for existence of /etc/systemd/system/"$(basename "${unit}")" before trying to mask.