add cpu_ubuntu16.04 dockerfile

[aimof]

I added a Dockerfile building the non-gpu environment from source. It seems to be required dockerfile to build from source because various changes is anticipated. Some people wants to use latest environments.

[TakuyaNarihira]

Thanks for your PR. It must be convenient for some people. We think we'd better wait for some opinions from other people about whether it deserves to be contained in the official repository. Thanks again for your suggestive PR.

[aimof]

Thanks. Honestly I was wondering if the place of dockerfile is right or not. Would you mind making "easy to use NNabla" Dockerfile Directory? I'm now coding Dockerfile containing nnabla_ext_cuda and Jupyter. It seems useful many people but I don't know which directory should i palce it.