Iterate local development with Docker

This makes a few changes to:

- Make local development consistent with our other apps. It's now
faster to start Celery locally since we don't try to build the
image each time - this is usually quick, but unnecessary.

- Add support for connecting to a local Redis instance. Note that
the previous suggestion of "REDIS = True" was incorrect as this
would be turned into the literal string "True".

I've also co-located and extended the recipes in the Makefile to
make them a bit more visible.

scripts/run_with_docker.sh

@@ -14,6 +14,8 @@ docker run -it --rm \
   -e AWS_ACCESS_KEY_ID=$AWS_ACCESS_KEY_ID \
   -e AWS_SECRET_ACCESS_KEY=$AWS_SECRET_ACCESS_KEY \
   -e SQLALCHEMY_DATABASE_URI=$SQLALCHEMY_DATABASE_URI \
+  -e REDIS_ENABLED=${REDIS_ENABLED:-0} \
+  -e REDIS_URL=${REDIS_URL:-''} \
   -v $(pwd):/home/vcap/app \
   ${DOCKER_IMAGE_NAME} \
   ${@}