From `pip install --help`:
```
-e, --editable <path/url> Install a project in editable mode (i.e.
setuptools "develop mode") from a local project path or a VCS url.
```
* 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.
- MacOS does not allow users to mount folders from the host unless it is
specified during build time.
- This is needed as the container on Mac needs to access the .bigchaindb
conf file residing on the host.
