When running gocast in a container, the default iptables implementation
may not match that used on the underlying host kernel. The current
container uses the legacy iptables implementation and calls the
`iptables` binary. This fails with exit code 3 when running on a host
using the newer nftables implementation. The container already has
`iptables-nft` binary included, so just needs a way to call this instead
of the default `iptables` binary.
This change implements a new `iptables_binary` config option, defaulting
to `iptables`, and calls this when adding or removing NAT rules.
Fixes#32
This change was written using AI LLM.
Authored-By: Claude Code (Sonnet 4.5)
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