Feature Request

Context

Docker running in some appliance (e.g., QNAP NAS') still uses legacy iptables.
This prevents fail2ban shipped with docker-mailserver from working properly, as it uses nftables.

Is your Feature Request related to a Problem?

Yes, the problem is "fail2ban in docker-mailserver won't work on systems still using legacy iptables (vs nftables)"

Describe the Solution you'd like

Add a new environment variable to switch to legacy iptables.

Are you going to implement it?

Yes, because I know the probability of someone else doing it is low, and I can learn from it.

What are you going to contribute??

PR is coming soon

Additional context

Alternatives you've considered

I tried to load nftable kernel module on my NAS. That won't be a general solution, but still, I learned that QNAP doesn't ship with a Linux kernel with nftable.

Who will that Feature be useful to?

Anyone using docker-mailserver on an appliance running docker, and not supporting modern firewall for Linux (QNAP NAS is an example, but there are more)

What have you done already?

PE is coming. I am not sure if I am respecting conventions used for this project. Happy to change my approach if needed.