Add a setup script

[kazlauskis]

It would be fantastic if there was a way to invoke a custom set up script once the drupal installation has finished. We do this to execute some composer/drush commands by mounting a script and manually triggering it but if there was a way to execute it automatically then it would be very useful. Something like this would suffice:

start.sh:

# ...

if [ -f /etc/set_up.sh ]; then
   bash /etc/set_up.sh
fi

tail -f /tmp/supervisord.log