Update Docker file to MXNet 1.6

[hassonofer]

Issue #, if available:

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Description of changes:

Update MXNet version to 1.6 on 2 Dockerfiles CPU and GPU. The PIP package already suggest the latest version of MXNet. This only to keep the containers and non Docker installations the same.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[hassonofer]

As this is my first contribution here, I didn't want to make a mess :)

But I think that the Dockerfile would be simpler if it pulled from Python instead of Ubuntu.