fix(Dockerfile): re-add ip6tables binary

When we reverted from Alpine 3.19 to Alpine 3.18 we somehow lost the
ip6tables package which caused issues for users of ipv6.

Dockerfile

@@ -25,6 +25,7 @@ FROM ${RUNTIME_BASE}
 
 RUN apk add --no-cache \
       iptables \
+      ip6tables \
       ipset \
       iproute2 \
       ipvsadm \
@@ -61,6 +62,10 @@ RUN if ! command -v iptables-nft > /dev/null; then \
         echo "ERROR: iptables-legacy is not installed" 1>&2; \
         exit 1; \
     fi && \
+    if ! command -v ip6tables > /dev/null; then \
+        echo "ERROR: ip6tables is not installed" 1>&2; \
+        exit 1; \
+    fi && \
     /iptables-wrapper-installer.sh --no-sanity-check
 
 WORKDIR /root