Tracking issue for further Docker improvements

[Oaphi]

#1430 is now merged, but there's still a list of further improvements to our setup that need attention:

• fix entrypoint.sh script to avoid the brittle /db-created file check;
• switch entrypoint.sh to rails db:prepare (blocked by script for creating the tag_paths view);
• share common image configuration between out-of-the-box Dockerfiles;
• add cron to the build step (needed to set up jobs);
  • install cron on build & set up whenever --update-crontab (check environment and output as well);
  • do not forget to add ENV.each { |k, v| env(k, v) } for the schedule;