Database file updates not reflected

Using a simple docker configuration like this:
```
docker run --rm \
  -p 8001:8001 \
  -v $(pwd):/mnt \
  datasetteproject/datasette \
  datasette serve -p 8001 -h 0.0.0.0 /mnt/database.db
```
If you update the `database.db` the changes are not reflected in the datasette queries. Running the same query using `sqlite3` inside the container works as expected.