Stop PMG from removing `source` line in `/etc/network/interfaces`

[William Edwards]

How do I stop PMG from removing the custom `source` line -which is supported according to the documentation- in `/etc/network/interfaces`?

Context:

I have some custom interfaces that PMG should not manage.

`/etc/network/interfaces` says:

# network interface settings; autogenerated
# Please do NOT modify this file directly, unless you know what
# you're doing.
#
# If you want to manage parts of the network configuration manually,
# please utilize the 'source' or 'source-directory' directives to do
# so.
# PVE will preserve these directives, but will NOT read its network
# configuration from sourced files, so do not attempt to move any of
# the PVE managed interfaces into external files!

The documentation at https://pmg.proxmox.com/pmg-docs/chapter-pmgconfig.html contains the following example (at "Example network setup /etc/network/interfaces"):

source /etc/network/interfaces.d/*

auto lo
iface lo inet loopback

auto ens18
iface ens18 inet static
        address  192.168.2.127
        netmask  255.255.240.0
        gateway  192.168.2.1

When I add this `source` line, PMG removes it on the next configuration change. Diff:

--- /etc/network/interfaces    2023-05-03 11:31:58.088046589 +0200
+++ /etc/network/interfaces.new    2023-05-03 11:30:47.487496517 +0200
@@ -9,8 +9,6 @@
 # configuration from sourced files, so do not attempt to move any of
 # the PVE managed interfaces into external files!
 
-source /etc/network/interfaces.d/*
-
 auto lo
 iface lo inet loopback
 
@@ -26,3 +24,4 @@
     gateway 2a0c:eb00:0:f5:185:233:174:1
     dns-nameservers 2a03:7900:2:0:31:3:104:61 2a03:7900:2:0:31:3:104:62
     dns-search prorelay.nl
+