[5.x] Add the $CRONTAB_SETTINGS variable

[shinsenter]

You can also easily set up cron jobs through the $CRONTAB_SETTINGS environment variable in the docker-compose.yml file.
When the container starts, these settings are loaded into crontab, giving you more flexibility to change them later.

services:
  web:
    image: shinsenter/php:8.3-fpm-nginx
    environment:
      ENABLE_CRONTAB: "1"
      CRONTAB_SETTINGS: |
        0 0 * * * echo "Hello new day!" | tee /tmp/cron-daily.txt
        * * * * * echo "This line will run every minute!" | tee /tmp/cron-every-minute.txt