Add IPv6 test.

builder/build-docker.sh

@@ -1,6 +1,8 @@
 #!/bin/bash -eu
 DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
 
+curl wtfismyip.com/text
+
 BUILD_OPTS="$*"
 
 DOCKER="docker"