Binder migration: New docker based setup with self-contained and stable pre-built base image on docker hub

[megies]

What this does..

• adds instructions and scripts to build a fully self-contained docker base image for binder
  • once everything is in place, base image should only need updating on major changes to seismo-live (e.g. when some notebooks need newer dependency versions, or new obspy released, etc...)
  • to include specific data(sets) for use in notebooks, the dockerfile should be further modified and the base image rebuilt
• modifies the root directory Dockerfile, which will fully rely on the above pre-built docker base image. only minimal steps should be in this file (like updating the notebooks in the base image)

The state of the PR can be tested on mybinder.org at this URL:

https://mybinder.org/v2/gh/krischer/seismo_live/binder_dockerfile

The only remaining issue is if/how to update notebooks when starting the docker base image. See the explanations at the bottom of README.md. Currently, the state of the notebooks will not be automatically updated when starting the docker image on binder. For debugging/testing, it is possible to $ git pull after starting a terminal in the jupyter session on binder. To update the notebooks, building a new base image seems easiest right now, although in principle the pre-notebook-start hooks (explained in README.md) should be able to do exactly that.

n.b. the PR commit history is rather messy due to a lot of CI debugging (without logs being available), but it might be useful in the future to retrace different approaches that were explored, so it might be good to keep it.

[megies]

Feel free to merge whenever @krischer