added tests

spec/lib/docker_container_mixin_spec.rb

@@ -3,9 +3,16 @@ require 'rails_helper'
 describe DockerContainerMixin do
   [:binds, :port_bindings].each do |method|
     describe "##{method}" do
+      let(:data) { [] }
+
       it 'is defined for Docker::Container' do
         expect(Docker::Container.instance_methods).to include(method)
       end
+
+      it 'returns the correct information' do
+        expect(CONTAINER).to receive(:json).and_return('HostConfig' => {method.to_s.camelize => data})
+        expect(CONTAINER.send(method)).to eq(data)
+      end
     end
   end
 end