Bugfix networking release

This is a very small bugfix where static IP in a container did not work using the fluent API due to the fact that docker needs to have --network on the container in order to create it. Now it will properly do so on the first available network added to it. The following proves the point:

      using (var nw = Fd.UseNetwork("unit-test-nw")
                        .UseSubnet("10.18.0.0/16").Build())
      {
        using (
          var container =
            Fd.UseContainer()
              .WithName("mycontainer")
              .UseImage("postgres:9.6-alpine")
              .WithEnvironment("POSTGRES_PASSWORD=mysecretpassword")
              .ExposePort(5432)
              .UseNetwork(nw)
              .UseIpV4("10.18.0.22")              
              .WaitForPort("5432/tcp", 30000 /*30s*/)
              .Build()
              .Start())
        {
          var ip = container.GetConfiguration().NetworkSettings.Networks["unit-test-nw"].IPAddress;
          Assert.AreEqual("10.18.0.22", ip);
        }
      }

Note that it will use the first network provided in UseNetwork as the --network parameter in docker create …. This solves the issue #81.

Cheers,
Mario