Prior to this commit, the docker action was using the method `prefix` on
an IPv4 and IPv6 address. This works fine for ruby versions 2.5 and
newer, however the ruby shipped with Vagrant is before 2.5, and
therefore the IPv4 and IPv6 classes do not have the prefix method,
resulting in an error. This commit fixes that by using a different
method of determining the prefix.
62b7e35169