Use systemctl commands inside of the image building process

[Andy3153]

I am attempting to "package" an old-ish (2018) native Linux proprietary program inside a container image, and as part of its install script, it runs some systemctl commands.

As far as I can understand, every time a RUN inside a Dockerfile is ran, it's being run as PID 1 of that container by default, so at first it doesn't seem possible.

Is there any solution? I really wouldn't want to have to rewrite the install script because it's pretty large.