Trying to access a value from a jsonencoded attribute in a terraform manifest file

Question

I have the following aws_ecs_task_definition resource definitions in a child module:

resource "aws_ecs_task_definition" "main" {
  family                   = "${var.name}-task-${var.environment}"
  network_mode             = "awsvpc"
  requires_compatibilities = ["FARGATE"]
  cpu                      = var.container_cpu
  memory                   = var.container_memory
  execution_role_arn       = aws_iam_role.ecs_task_execution_role.arn
  task_role_arn            = aws_iam_role.ecs_task_role.arn
  container_definitions = jsonencode([{
    name        = "${var.name}-container-${var.environment}"
    image       = "${var.container_image}:latest"
    essential   = true
    environment = var.container_environment
    portMappings = [{
      protocol      = "tcp"
      containerPort = var.container_port
      hostPort      = var.container_port
    }]
    logConfiguration = {
      logDriver = "awslogs"
      options = {
        awslogs-group         = aws_cloudwatch_log_group.main.name
        awslogs-stream-prefix = "ecs"
        awslogs-region        = var.region
      }
    }
    secrets = var.container_secrets
  }])

  tags = {
    # Name        = "${var.name}-task-${var.environment}"
    Environment = var.environment
    Name = "genesis-ecs-task-definition"
    Manager = "terraform"
  }
}

And in that child module I have the following in the outputs.tf file:

output "container_image" {
    value = jsondecode(aws_ecs_task_definition.main.container_definitions)[0]["image"]
}

I want to output the value of the container_definitions image value only in the parent module but my solutions don't work as I get a blank output:

container_image = {}

Any pointers on how to do this?