provision.sh: default to pulling image from docker hub

Use CUSTOM_IMAGE variable to decide whether or not
to build the image from a Dockerfile or to pull in
from Docker hub repo.

The latest-bro-sandbox image on Docker hub is now up to date
so this should be used first unless one wants customizations.
Plus, it's much faster than creating an image from scratch because
the Bro build process is consuming.

Options:
yes - Use $DOCKERFILE if it exists, otherwise fallback to hub image
no  - Use Docker hub image, pulled from servers