[Docker] Added a check to make sure an argument is passed

[digininja]

PR to go with issue #6206 to stop unbound variable errors when running docker-entrypoint.sh without an argument.

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[digininja]

Yes, I can add that.

[digininja]

I've done that, and just for OCD value, put the commands into alphabetical order.

My only worry with the change I've made is that the final else on the original if passes through any commands the script doesn't know about to exec. Should this be able to be reached without an argument? If it should, then my update will break that functionality as I exit early if no command is passed. If this has to have at least one argument, then my change is OK.

[wing328]

I think we can go with what you've right now and submit PRs later if there are feedbacks from other users on the usage.