Check ipv6 docker support

Makefile

@@ -1,5 +1,10 @@
-prepare-test:
+check-docker:
+	@python check_docker_ipv6_support.py
+
+build-test-image:
 	@docker build -f Dockerfile-test-healthcheck -t docker-dns:test-healthcheck-1.0 .
 
+prepare-test: build-test-image check-docker
+
 test:
 	@python -m tests

check_docker_ipv6_support.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+
+import docker
+import sys
+from ipaddress import ip_network, IPv6Network
+
+client = docker.from_env()
+
+for network in client.networks.list():
+    subnets = network.attrs['IPAM']['Config']
+    for subnet in subnets:
+        address = ip_network(subnet['Subnet'])
+        if isinstance(address, IPv6Network):
+            print("Docker has ipv6 support")
+            sys.exit(0)
+
+print("Docker has no ipv6 support")
+sys.exit(1)