Mark entire CudaTest class as requiring docker

The call to get_docker_client in setUpClass will fail if docker is not
available on the host.

test/test_nvidia.py

@@ -250,6 +250,7 @@ def test_nvidia_env_subs(self):
             p.get_environment_subs(mock_cliargs)
         self.assertEqual(cm.exception.code, 1)
 
+@pytest.mark.docker
 class CudaTest(unittest.TestCase):
     @classmethod
     def setUpClass(self):
@@ -286,15 +287,13 @@ def setUp(self):
         em.Interpreter._wasProxyInstalled = False
 
 
-    @pytest.mark.docker
     def test_no_cuda(self):
         for tag in self.dockerfile_tags:
             dig = DockerImageGenerator([], {}, tag)
             self.assertEqual(dig.build(), 0)
             self.assertNotEqual(dig.run(), 0)
             dig.clear_image()
 
-    @pytest.mark.docker
     def test_cuda_install(self):
         plugins = list_plugins()
         desired_plugins = ['cuda']