ci: extract enabling ipv6 on docker into a script

src/ci/scripts/enable-docker-ipv6.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+# Looks like docker containers have IPv6 disabled by default, so let's turn it
+# on since libstd tests require it
+
+set -euo pipefail
+IFS=$'\n\t'
+
+source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
+
+if isLinux; then
+    sudo mkdir -p /etc/docker
+    echo '{"ipv6":true,"fixed-cidr-v6":"fd9a:8454:6789:13f7::/64"}' | sudo tee /etc/docker/daemon.json
+    sudo service docker restart
+fi