Whoever said you have to be a soldier to be disciplined could not be more wrong. Yes soldiers are very well disciplined, but only because they have been very well practised in doing simple tasks over and over again.
Take Kapooka basic training for example. As a new recruit, you are required to complete basic tasks every day. As a want-to-be soldier you are more focused on completing the task to your very best standard. This will stop you and the platoon from being punished with extra duties (usually the same task again). However, very intelligently the army is grooming you, injecting you with a consistent dose of daily discipline.