add array index to fix bug in fallback value for container definitions argument

Author: dragonfleas

ecs_task_definitions.tf

@@ -1,7 +1,7 @@
 
 resource "aws_ecs_task_definition" "service" {
   family                   = var.service_name
-  container_definitions    = var.container_definitions != null ? var.container_definitions : "[${module.service_container_definition.json_map_encoded}]"
+  container_definitions    = var.container_definitions != null ? var.container_definitions : "[${module.service_container_definition[0].json_map_encoded}]"
   execution_role_arn       = aws_iam_role.ecs_task_execution_role.arn
   task_role_arn            = aws_iam_role.ecs_task_role.arn
   network_mode             = "awsvpc"