Add cron to official Docker image
So long as the crontab is being generated, use it for a nice out-of-box experience with Docker and search.

Created fresh test instance docker-compose with equivalent changes to this review request.
With the modified docker-entrypoint.sh in the same dir:
Dockerfile
FROM beanbag/reviewboard:4.0.2
RUN apt-get update && apt-get install -y cron
ADD docker-entrypoint.sh /


docker-compose.yml based on docker-compose.mysql.yaml
...
  reviewboard:
    build: .
    environment:
      - REVIEWBOARD_CRONTAB=/site/conf/cron.conf
...


Start instance, and create search index with docker-compose exec reviewboard /bin/bash -c 'gosu reviewboard rb-site manage /site rebuild_index'

Wait until the next cron run, and check the modified times in the search index e.g. docker-compose exec reviewboard /bin/bash -c 'ls -al /site/data/search-index/'