Updated Docker docs for new images (horovod#2949)

Signed-off-by: Travis Addair <[email protected]>

Author: tgaddair

docs/docker.rst

@@ -3,25 +3,11 @@
 Horovod in Docker
 =================
 
-To streamline the installation process on GPU machines, we have published the reference `Dockerfile <https://github.com/horovod/horovod/blob/master/Dockerfile.gpu>`__ so
-you can get started with Horovod in minutes. The container includes `Examples <https://github.com/horovod/horovod/tree/master/examples>`__ in the ``/examples``
+To streamline the installation process, we have published reference `Dockerfiles <https://github.com/horovod/horovod/blob/master/docker>`__ so
+you can get started with Horovod in minutes. These containers include Horovod `examples <https://github.com/horovod/horovod/tree/master/examples>`__ in the ``/examples``
 directory.
 
-Pre-built Docker containers with Horovod are available on `DockerHub <https://hub.docker.com/r/horovod/horovod>`__.
-
-
-Building
-~~~~~~~~
-Before building, you can modify ``Dockerfile.gpu`` to your liking, e.g. select a different CUDA, TensorFlow or Python version.
-
-.. code-block:: bash
-
-    $ mkdir horovod-docker-gpu
-    $ wget -O horovod-docker-gpu/Dockerfile https://raw.githubusercontent.com/horovod/horovod/master/Dockerfile.gpu
-    $ docker build -t horovod:latest horovod-docker-gpu
-
-For users without GPUs available in their environments, we've also published a `CPU Dockerfile <https://github.com/horovod/horovod/blob/master/Dockerfile.cpu>`__
-you can build and run similarly.
+Pre-built Docker containers with Horovod are available on `DockerHub <https://hub.docker.com/r/horovod/horovod>`__ for GPU, CPU, and `Ray <https://ray.io>`__.
 
 
 Running on a single machine