Superset new requires a SECRET_KEY environment variable

[Barkha]

The docker instructions for SuperSet need to be changed
from:
docker run
--network pinot-demo
--name=superset
-p 8088:8088
-d apachepinot/pinot-superset:latest
to:
docker run
--network pinot-demo
--name=superset
-p 8088:8088
-e SUPERSET_SECRET_KEY=
-d apachepinot/pinot-superset:latest