updated docker image to fixed version 1.15.2 and mnist convolutional …

[hmeiland]

…script to v1.7.0, since latest and master are now working

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Description

update to fixed version of docker image and job script; since latest / master versions are not working/available.

[cdeil]

@alfpark - merge?

I was trying out the batch-shipyard tutorial at https://batch-shipyard.readthedocs.io/en/latest/02-batch-shipyard-quickstart/ and ran into the issue that's fixed here that https://raw.githubusercontent.com/tensorflow/models/master/tutorials/image/mnist/convolutional.py doesn't exist any more.