document how to use driftctl with fargate scheduled runs (cron)

[sjourdan]

Scenario: AWS Fargate is used as a scheduler to run driftctl at regular intervals (say, @daily or @hourly).
Goal: provide documentation about how to do it properly, so users can do it easily

Source: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/scheduled_tasks.html

Terraform ECS Task Definition: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/ecs_task_definition