Java app can't connect to rabbitMQ from the same docker container

I have a sample Spring Boot app that produce message:

@Scheduled(fixedRate = 10000)
public void test() throws Exception {
    ConnectionFactory factory = new ConnectionFactory();
    factory.setHost("localhost");
    try (Connection connection = factory.newConnection();
         Channel channel = connection.createChannel()) {
        channel.queueDeclare(QUEUE_NAME, false, false, false, null);
        String message = "Hello Rabbit!";
        channel.basicPublish("", QUEUE_NAME, null, message.getBytes(StandardCharsets.UTF_8));
        System.out.println(" [x] Sent '" + message + "'");
    }
}

It is dockerized:

FROM openjdk:11 as jdkbase
FROM jdkbase

COPY target/RabbitMQtester-0.0.1-SNAPSHOT.jar /run/RabbitMQtester-0.0.1-SNAPSHOT.jar
#ADD target/sfida-1.0.0.jar /run/sfida-1.0.0.jar

ENTRYPOINT java -Dfile.encoding=UTF-8 -jar /run/RabbitMQtester-0.0.1-SNAPSHOT.jar

I need to build an image containing both RabbitMQ and my app. Here is docker-compose.yml for this task:

version: "3.7"
services:
  mytestapp:
    build: .
  rabbitmq:
    image: rabbitmq:3.8.3-management-alpine
    hostname: localhost
    environment:
      RABBITMQ_DEFAULT_USER: guest
      RABBITMQ_DEFAULT_PASS: guest
    ports:
      - 5672:5672
      - 15672:15672

When i start the resulting image, i can access rabbitMQ admin page from my host. Also, i can run my app from the host and it'll access rabbitMQ. But my app, which is in the container/image, can't access RabbitMQ (it throws "Connection refused (Connection refused)" errors) What is wrong with my configurations?

UPD: I've also tried to change docker-compose.yml to this (and connection in my app to factory.setHost("rabbitmq"); )

version: "3.8"
services:
  mytestapp:
    build: .
    networks:
      - some-net
  rabbitmq:
    image: rabbitmq:3.8.3-management-alpine
    hostname: rabbitmq
    networks:
      - some-net
    environment:
      RABBITMQ_DEFAULT_USER: guest
      RABBITMQ_DEFAULT_PASS: guest
    ports:
      - 5672:5672
      - 15672:15672
networks:
  some-net:
    driver: bridge

and to this:

version: "3.8"
services:
  mytestapp:
    build: .

  rabbitmq:
    image: rabbitmq:3.8.3-management-alpine
    hostname: rabbitmq
    environment:
      RABBITMQ_DEFAULT_USER: guest
      RABBITMQ_DEFAULT_PASS: guest
    ports:
      - 5672:5672
      - 15672:15672

but result is the same...