Docker image could have cron job for running artisan schedule:run

[stefaro]

Is your feature request related to a problem? Please describe.
Setting up cron job for running schedule:run is cumbersome in docker image.

Describe the solution you'd like
I think there should be a cron job in docker image which would be controlled by an environment variable. A good default value for the value would be * * * * *, so that scheduled tasks work out of the box.
Installing the cron job would happen in startup.sh. Cron is already installed in the image, we just need to probably start it in the background. Starting cron daemon would also happen in startup.sh.

Describe alternatives you've considered
Environment value controlling the cron job could also be unset by default, and so the cron job wouldn't be installed at all.

Additional context
I think I can provide a pull request with these changes implemented, if this is concidered as a good idea.

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