build: Fix docker script to always update the volumes (#171)

- Noticed since switching to a docker-compose, that changes I made to
DEMO.md weren't being copied to the image if I hadn't made any other
changes and the image still existed
- Instead of doing a docker-compose down after and removing the image,
just add a volume for the data directory as well to always get data
instead of using data from a previous container
- Tested by running `npm run docker`, closing, then changing the
DEMO.md, running `npm run docker` again, and checking my changes to
DEMO.md were displayed
- Also tested with a docker-compose.override.yml with a path to a folder
for my own data as detailed in the README, and that worked correctly as
well

docker-compose.yml

@@ -7,3 +7,5 @@ services:
       pull: true
     ports:
       - '${DEEPHAVEN_PORT:-10000}:10000'
+    volumes:
+      - ./docker/data/:/data