fix: remove apk add iptables-nft — built-in on Alpine 3.18+

In Alpine 3.18+, the 'iptables' package IS the nftables variant.
iptables-nft is not a separate package anymore — just need to flip
update-alternatives from iptables-legacy to iptables-nft.

vpn/Dockerfile

@@ -3,8 +3,7 @@
 FROM ghcr.io/wg-easy/wg-easy:latest
 
 # The upstream image defaults to iptables-legacy via update-alternatives.
-# Switch to iptables-nft so it works on kernels where only nftables
-# netfilter modules are available (iptable_nat module missing).
-RUN apk add --no-cache iptables-nft && \
-    update-alternatives --set iptables /usr/sbin/iptables-nft && \
+# Switch iptables to the nftables backend (already provided by the 'iptables'
+# package on Alpine 3.18+). No apk add needed — iptables-nft is built-in.
+RUN update-alternatives --set iptables /usr/sbin/iptables-nft && \
     update-alternatives --set ip6tables /usr/sbin/ip6tables-nft