Run Laravel schedule in docker image using supervisord (#6606)

* docker: Rename /entrypoint.sh to /startup.sh

This script is not configured as the docker image ENTRYPOINT, thus it is
misleading to name it so.

* docker: Terminate supervisord if a process enters the FATAL state

By terminating PID 1, this will also terminate the Docker container.

* docker: Use supervisord to start up apache and cron

Note that this uses `apache2ctl -DFOREGROUND` rather than manually
sourcing /etc/apache2/envvars and running apache2, as recommended at
https://advancedweb.hu/2018/07/03/supervisor_docker/.

* docker: Add artisan schedule:run to crontab

This also switches to executing /var/www/html/artisan directly.

* docker: Run artisan schedule:run directly from supervisor

This has the following benefits over using cron:
- Cron doesn't need to be installed
- Docker-provided environment variables are preserved
- It's easy and explicit to run as the docker user

docker/supervisord.conf

@@ -0,0 +1,27 @@
+[supervisord]
+nodaemon=true
+
+[program:apache]
+; https://advancedweb.hu/2018/07/03/supervisor_docker/
+command=apache2ctl -DFOREGROUND
+killasgroup=true
+stopasgroup=true
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
+
+[program:run_schedule]
+; Simply run the Laravel command scheduler every minute
+command=/bin/bash -c "while true; do /var/www/html/artisan schedule:run; sleep 1m; done"
+user=docker
+stdout_logfile=/dev/stdout
+stdout_logfile_maxbytes=0
+stderr_logfile=/dev/stderr
+stderr_logfile_maxbytes=0
+
+
+; https://stackoverflow.com/a/37527488/119527
+[eventlistener:exit_on_any_fatal]
+command=supervisor-exit-event-listener
+events=PROCESS_STATE_FATAL