Init docker for local development environment. (#4193)

This commit will try to dockerize superset in local development
environment.

The basic design is:
- Enable superset, redis and postgres service instead of using sqlite,
  just want to simulate production environment settings
- Use environment variables to config various app settings. It's easy to
  run and config superset to any environment if we use environment than
  traditional config files
- For local development environment, we just expose postgres and redis
  to local host machine thus you can connect local port via `psql` or
  `redis-cli`
- Wrap start up command in a standard `docker-entrypoint.sh`, and use
  `tail -f /dev/null` combined with manually `superset runserver -d` to
  make sure that code error didn't cause the container to fail.
- Use volumes to share code between host and container, thus you can use
  your favourite tools to modify code and your code will run in
  containerized environment
- Use volumes to persistent postgres and redis data, and also
  `node_modules` data.
  - If we don't cache `node_modules` in docker volume, then every time
    run docker build, the `node_modules` directory, will is about 500 MB
    large, will be sent to docker daemon, and make the build quite slow.
- Wrap initialization commands to a single script `docker-init.sh`

After this dockerize setup, any developers who want to contribute to
superset, just follow three easy steps:

```
git clone https://github.com/apache/incubator-superset/
cd incubator-superset
cp contrib/docker/{docker-build.sh,docker-compose.yml,docker-entrypoint.sh,docker-init.sh,Dockerfile} .
cp contrib/docker/superset_config.py superset/
bash -x docker-build.sh
docker-compose up -d
docker-compose exec superset bash
bash docker-init.sh
```

docs/installation.rst

@@ -35,6 +35,27 @@ The Superset web server and the Superset Celery workers (optional)
 are stateless, so you can scale out by running on as many servers
 as needed.
 
+Start with Docker
+-----------------
+
+If you know docker, then you're lucky, we have shortcut road for you to 
+initialize development environment: ::
+
+    git clone https://github.com/apache/incubator-superset/
+    cd incubator-superset
+    cp contrib/docker/{docker-build.sh,docker-compose.yml,docker-entrypoint.sh,docker-init.sh,Dockerfile} .
+    cp contrib/docker/superset_config.py superset/
+    bash -x docker-build.sh
+    docker-compose up -d
+    docker-compose exec superset bash
+    bash docker-init.sh
+
+After several minutes for sueprset initialization to finish, you can open a 
+a browser and view `http://localhost:8088` to start your journey.
+
+Or if you're curious and want to install superset from bottom up, then go 
+ahead.
+
 OS dependencies
 ---------------