Not the maintainer here, but cf.:
On Tue, Aug 08, 2017 at 02:29:39PM +0200, jakob-tobias.win...@stuttgart.de
> Package: varnish
> Version: 5.0.0-7+deb9u1
> Severity: important
> When using systemd its hard to change the port varnish listens on. In our
> case we want it to listen on port 80.
> You have to edit /lib/systemd/system/varnish.service as all other config
> options I found are simply ignored.
> In addition on update of the varnish package the systemd unit file gets
> overwritten and without notice or warning you have a broken varnish with
> possible external effect.
> Please declare the unitfile as config file so the update process asks what
> to do if there are changes to the file or make changing the port in
> /etc/default/varnish possible.
# Note: If systemd is installed, this file is obsolete and ignored. You will
# need to copy /lib/systemd/system/varnish.service to /etc/systemd/system/ and
# edit that file.