Don't start service on installation of Debian package

A clear difference between Debian/Ubuntu and for example Red Hat/Fedora is that packages which include system services will enable and start those services at install time in Debian/Ubuntu whereas they will not start automatically in Red Hat/Fedora.

Sometimes it would be very convenient if the service would not start automatically, for example if you need to configure the service before starting it for the first time.

To prevent the automatic start of system services at install time in Debian, just set the RUNLEVEL environment variable like so:

RUNLEVEL=1 apt install -y PKG_NAME

Then you are free to configure your system before you start the service for real:

systemctl enable PKG_NAME
systemctl start PKG_NAME