Cross-built docker image with upgrade#10

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liuqun wants to merge 4 commits intoromilly:masterfrom

liuqun:crossbuild

Commits on Jul 25, 2018

Upgrade pip, /etc/pip.conf, numpy and tensorflow packages
Liu Qun
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Dockerfile: Rebase to official multiarch image
Liu Qun
committed
docs: Update cross-build instructions in README
Liu Qun
committed

Commits on Jul 26, 2018

Debian==stretch && python==2.7.x && numpy==1.14.5
Liu Qun
committed