I am using a custom milter to prevent spam and server probing. Currently I need to fork the base image and patch master.cf to get my milter included.
Add some option to setup.sh to add custom SMTP milter IP/DNS.
None.
Users with custom SMTP milters.
None.
I am not sure about this one, but can't you use user-patches.sh to patch master.cf?
I agree with @aendeavor that a patch from user-patches.sh makes the most sense. This is exactly the kind of thing it is there for.
Thanks for the hint! I think this is a valid solution for my problem and I will modify master.cf that way.