pin mxnet/python image version

[mayankjuneja]

The mxnet/python image version was updated on docker hub, the worker pods in the example study fail to start. mxnet folder seems to be missing from / in the latest image, I see the following error in the logs -
python: can't open file '/mxnet/example/image-classification/train_mnist.py': [Errno 2] No such file or directory

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[mayankjuneja]

I signed it!

[mayankjuneja]

/check-cla

[jlewi]

/ok-to-test

[jlewi]

/assign @YujiOshima

[YujiOshima]

/lgtm

[mayankjuneja]

/check-cla

[googlebot]

CLAs look good, thanks!

[YujiOshima]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: YujiOshima

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[YujiOshima]

@mayankjuneja Thanks!