Update docker_ubuntu.md to recommend a compatible version of ubuntu

[GuillaumeLeclerc]

Numpy now require python 3.6, which is not available without extra ppa on ubuntu 16.04. Updating the documentation to make sure that we recommend a version that works with the Dockerfile provided

[rajat2004]

I think the Dockerfiles and the python script should also be updated to use 18.04 by default

[ghost]

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