* Basing BigchainDB docker image from `python:3.6` in `Dockerfile` and `Dockerfile-dev`.
* Added ENV var for websocket interface
* Optimization to Dockerfile-dev to save build time during dev/test.
Pre-installing the `pynacl` dependeny in separate command so that devs
do not need to wait while testing. The trade-off is that devs will
need to clear (`rmi`) the image to start from scratch.
* Include configuration step in Dockerfile
* Document dev setup with docker-compose
* Add server API env vars and port
* Parameterize SERVER_BIND and API_ENDPOINT env vars
* Give a bit more detailed information on GET /
