Setting IPv6 subnet without IPv4 in docker_network triggers recreate

[mattlqx]

Cookbook version

2.17.0

Chef-client version

13.6.0

Platform Details

Ubuntu 16.04

Scenario:

I noticed while running the tests for #952 that a recreate is triggered for the IPv6 network test. This is because the resource only specifies an IPv6 subnet. (https://github.com/chef-cookbooks/docker/blob/v2.17.0/test/cookbooks/docker_test/recipes/network.rb#L233-L237) Docker will auto-assign an IPv4 subnet to the network if one isn't specified and on subsequent Chef runs, the network state is loaded and is seen to have the extra IPv4 subnet triggering a recreate.

  * docker_network[network_ipv6] action create
    - deleting network_ipv6
    - update network_ipv6
    -   set subnet      to "fd00:dead:beef::/48" (was ["172.24.0.0/16", "fd00:dead:beef::/48"])

Steps to Reproduce:

Run kitchen converge network twice and observe the recreation of the network.

Expected Result:

An up-to-date resource on subsequent runs.

Actual Result:

The network is destroyed and recreated.