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

@@ -2,8 +2,8 @@
 # Fixes crash-loop when host kernel lacks legacy iptable_nat module.
 FROM ghcr.io/wg-easy/wg-easy:latest
 
-# The upstream image registers only iptables-legacy with update-alternatives.
-# iptables-nft binary exists but isn't registered as an alternative key.
-# Override the alternatives-managed symlinks directly.
-RUN ln -sf /usr/sbin/iptables-nft /usr/sbin/iptables && \
-    ln -sf /usr/sbin/ip6tables-nft /usr/sbin/ip6tables
+# The upstream image defaults to iptables-legacy via update-alternatives.
+# 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