Unable to get a response from a container on the same network, Docker compose, Mountebank

I have a Go application and three docker containers for the app, database and mountebank to mock/stub HTTP response.

I would like my mountebank container to return an appropriate response when the test_api (app container) makes a request.

The mountebank container returns a proper response when I call the endpoint using Postman.

However, when my code inside the test_api container send GET request like the code below It always receives

dial tcp 127.0.0.1:3000: connect: connection refused

Here is how test_api send a request to mountebank container

func getSuper(id string) (*float64, error) {
    var url = "http://localhost:3000/ato/employee/?/balance"
    url = strings.Replace(url, "?", id, 1)
    log.Println("Sending request to this url: ", url)
    resp, err := http.Get(url)
    if err != nil {
        return nil, &errorhandling.RequestError{Context: "getSuper calling ato api", Code: errorhandling.Internal, Message: err.Error()}
    }
    body, err := resposeToByte(resp)
    if err != nil {
        log.Println("err in coverting response to byte[]:", err)
        return nil, &errorhandling.RequestError{Context: "getsuper resposeToByte", Code: errorhandling.Internal, Message: err.Error()}
    }
    superData, err := UnmarshalSuperDetails(body)
    if err != nil {
        return nil, &errorhandling.RequestError{Context: "UnmarshalSuperDetails())", Code: errorhandling.Internal, Message: err.Error()}
    }
    log.Println("super details ", superData)

    return &superData.SuperBalance, nil
}

here is my docker-compose

version: '3.7'

services:
  db:
    container_name: "test_db"
    platform: linux/x86_64
    build:
      context: .
      dockerfile: db.Dockerfile
    networks:
      - default
    restart: always
    ports:
      # <Port exposed> : < MySQL Port running inside container>
      - "3306:3306"
    # setting some env vars to create the DB
    environment:
      MYSQL_RANDOM_ROOT_PASSWORD: "secret"
      MYSQL_DATABASE: "IGD"
      MYSQL_USER: "tester"
      MYSQL_PASSWORD: "secret"
      # OR if you want to use "root" as the user, just these two lines
      # MYSQL_ROOT_PASSWORD: ${DATABASE_PASSWORD}
      # MYSQL_DATABASE: ${DATABASE_NAME}

    # we mount a data volume to make sure we don't lose data
    volumes:
      - mysql_data:/var/lib/mysql
    command: --default-authentication-plugin=mysql_native_password
  api:
    container_name:  "test_api"
    # we want to use the image which is build from our Dockerfile
    build:
      context: .
      dockerfile: api.Dockerfile
    ports:
      - "8080:8080"
    # we are depending on the mysql backend
    depends_on:
      - db
    # We mount the working dir into the container, handy for development
    # This is what makes the hot reloading work inside of a Docker container
    volumes:
      - .:/app/  
  mountebank:
    container_name: mountebank
    image: jkris/mountebank:latest
    build:
      context: .
      dockerfile: mb.Dockerfile
    volumes:
    - ./imposters:/imposters
    ports:
    - 2525:2525
    - 3000:3000
    #- 8090:8090
    command: --configfile /imposters/imposter.json --allowInjection
    networks:
      - default
networks:
  default:
volumes:
  mysql_data:

my imposter:

 { "port": 3000,
  "protocol": "http",
  "stubs": [
    {
      "predicates": [
        {
          "equals": {"path":"/ato/employee/a/balance"}
        }
      ],
      "responses": [
        {
          "is": {
            "statusCode": 404,
            "headers": {
              "Content-Type": "application/json"
            },
            "body": {"error": "value not available"}
          }
        }
      ]
    },
    {
      "predicates": [
        {
          "equals": {"path":"/ato/employee/58957fc7-4e24-44e5-9eb1-a7fa0297613b/balance"}
        }
      ],
      "responses": [
        {
          "is": {
            "statusCode": 200,
            "headers": {
              "Content-Type": "application/json"
            },
            "body": { "employeeId":"50ccf5d6-2056-4e0c-a160-4e51638410c7",
                      "superBalance":1050.0}
          }
        },
continues...

Here is my docker file for mountebank

FROM alpine:3.14

ENV MOUNTEBANK_VERSION=2.4.0

RUN apk add --update nodejs-lts && \
    apk add --update npm
RUN npm install -g mountebank@${MOUNTEBANK_VERSION} --production

EXPOSE 2525
ENTRYPOINT ["mb"]
CMD ["start"]

How can I make a change in my code in order to make test_api receive a proper response from the mountebank?

You should change following line in your test_api;

var url = "http://localhost:3000/ato/employee/?/balance"

With the following one;

var url = "http://mountebank:3000/ato/employee/?/balance"

Since these are different container and you should specify their name or IPs in Docker environment. Your test_api requests to its localhost and there is no open port for 3000 and you would get connection refused error. You can take a look Docker Networking for further information.