binder-compatible docker image

[nthiery]

binder is a neat web service that gives interactive access to any GitHub repository containing Jupyter notebooks. Providing a docker image for SageMath that can be used in binder would bring this great feature to the SageMath community.

See also: binder-project/binder#90

[nthiery]

The beta mybinder version enables arbitrary Dockerfile. Here is an example repo using this:
https://github.com/nthiery/test-binder-sage

The Dockerfile there is pretty short. So at the end of the day we probably don't need a specific binder-enabled container. Instead we could lift whatever can be (installing additional kernels? upgrading jupyter?) to the base SageMath image), and provide an example/template repo.

[nthiery]

Done in 069a8e0, with template repo at https://github.com/sagemath/sage-binder-env/