resolve service hostname in ECS fargate task

I am trying to automate my ECS fargate cluster making using terraform.

I have a SpringBoot project with microservices containerized, and I am putting these images in a single task definition for an ECS service for the backend.

The ECS cluster is initially running, but Kafka is getting stopped with the error :

ERROR Unable to resolve address: zookeeper:2181
(org.apache.zookeeper.client.StaticHostProvider)

I have given the image for zookeeper(bitnami) and used the KAFKA_CFG_ZOOKEEPER_CONNECT environment variable too.

EDIT : My task definition:

resource "aws_ecs_task_definition" "this" {
  family                   = local.application_name
  requires_compatibilities = [local.launch_type]
  execution_role_arn       = data.aws_iam_role.ecs_task_execution_role.arn
  network_mode             = "awsvpc"
  cpu                      = "4096"
  memory                   = "30720"
  container_definitions = jsonencode([
    {
      name      = "zookeeper"
      image     = "docker.io/bitnami/zookeeper:latest"
      essential = true
      cpu       = 512
      memory    = 1024
      portMappings = [
        {
          containerPort = 2181
          hostPort      = 2181
        }
      ]
      "environment" : [
        { "name" : "ALLOW_ANONYMOUS_LOGIN", "value" : "yes" },
        { "name" : "ZOO_LISTEN_ALLIPS_ENABLED", "value" : "yes" }
      ]
    },
    {
      name      = "kafka-server"
      image     = "docker.io/bitnami/kafka:latest"
      essential = true
      cpu       = 512
      memory    = 1024
      portMappings = [
        {
          containerPort = 9092
          hostPort      = 9092
        }
      ]
      depends_on =  [
        "zookeeper"
      ]
      "environment" : [
        { "name" : "KAFKA_CFG_ZOOKEEPER_CONNECT", "value" : "zookeeper:2181" },
        { "name" : "ALLOW_ANONYMOUS_LOGIN", "value" : "yes" },
        { "name" : "ALLOW_PLAINTEXT_LISTENER", "value" : "yes" }
      ],
    },
    {
      name      = "email-service"
      image     = "my-email-image"
      essential = true
      cpu       = 512
      memory    = 1024
      portMappings = [
        {
          containerPort = 8090
          hostPort      = 8090
        }
      ]
      "environment" : [
        { "name" : "EMAIL_URL", "value" : "email-service" },
        { "name" : "EMAIL_PORT", "value" : "8090" },
        { "name" : "KAFKA_URL", "value" : "kafka-server" },
        { "name" : "KAFKA_PORT", "value" : "9092" },
      ]
    }
  ])
}