Building the final image results in error

In your example [here](https://github.com/aws/sagemaker-chainer-container) for building the final container, you execute this command `docker build -t preprod-chainer:4.1.0-gpu-py3 -f docker/4.1.0/final/py3/Dockerfile.gpu .`. However the build fails with the following error.

Step 9/12 : RUN pip3 install --no-cache /sagemaker_chainer_container-1.0-py2.py3-none-any.whl

 ---> Running in 03d3d1d3f888

Processing /sagemaker_chainer_container-1.0-py2.py3-none-any.whl

Requirement already satisfied: numpy>=1.14 in /usr/local/lib/python3.5/dist-packages (from sagemaker-chainer-container==1.0) (1.15.4)

Collecting retrying==1.3.3 (from sagemaker-chainer-container==1.0)

  Downloading https://files.pythonhosted.org/packages/44/ef/beae4b4ef80902f22e3af073397f079c96969c69b2c7d52a57ea9ae61c9d/retrying-1.3.3.tar.gz

Collecting sagemaker-containers>=2.2.5 (from sagemaker-chainer-container==1.0)

  Downloading https://files.pythonhosted.org/packages/be/7f/d5011a5d24d86872813140a31d9b8f6d38395a5937c917b9cff1c16e2a6d/sagemaker_containers-2.4.0.tar.gz (43kB)

Collecting chainer==5.0.0 (from sagemaker-chainer-container==1.0)

  Downloading https://files.pythonhosted.org/packages/bd/34/be31d10ff7f6a9452025866a6d515e1fbc877ff2ee68d9c7197c75f15797/chainer-5.0.0.tar.gz (510kB)

    Complete output from command python setup.py egg_info:

    

    We detected that ChainerMN is installed in your environment.

    ChainerMN has been integrated to Chainer and no separate installation

    is necessary. Please uninstall the old ChainerMN in advance.

A quick and dirty fix is to add this line to the `Dockerfile` before [line 18](https://github.com/aws/sagemaker-chainer-container/blob/master/docker/4.1.0/final/py3/Dockerfile.gpu#L18)

```
RUN pip3 uninstall chainermn -y
RUN pip3 install --no-cache /sagemaker_chainer_container-1.0-py2.py3-none-any.whl
```
Note the error is coming from [here](https://github.com/chainer/chainer/blob/master/setup.py#L98).

Note that I am building against `4.1.0` but in the logs above, chainer is `5.0.0`.

Note that the wheel is `dist/sagemaker_chainer_container-1.0-py2.py3-none-any.whl`, which was created by your instructions `python setup.py bdist_wheel`.

I haven't tested yet, as I am still mid-way through your [README.rst](https://github.com/aws/sagemaker-chainer-container/blob/master/README.rst).

Note this may impact all your final Dockerfiles.

Start by reproducing the README.rst build command for docker/4.1.0/final/py3/Dockerfile.gpu and inspect the other final Dockerfiles for the same dependency setup. Read the referenced Chainer setup.py error and verify the wheel installation step. Done means the affected final Docker image builds successfully without the ChainerMN conflict.

Written by the indexing model from the issue text.