Configure nofiles ulimit in Terraform ECS task (#3249)

The low default ulimit `nofiles` value (`4096`) in Fargate was observed
to cause errors when running with a large number of hosts and a small
number of servers. Each server should be able to server more than 4096
simultaneous clients.

tools/terraform/ecs.tf

@@ -147,6 +147,13 @@ resource "aws_ecs_task_definition" "backend" {
             awslogs-stream-prefix = "fleet"
           }
         },
+        ulimits = [
+          {
+            name      = "nofile"
+            softLimit = 999999
+            hardLimit = 999999
+          }
+        ],
         secrets = [
           {
             name      = "FLEET_MYSQL_PASSWORD"
@@ -352,4 +359,4 @@ output "fleet_ecs_cluster_arn" {
 
 output "fleet_ecs_cluster_id" {
   value = aws_ecs_cluster.fleet.id
-}
+}