Connection failed to mysql from docker compose service

Docker Engine Compose
1

I’m a Docker newbie and I’m trying to compose a .NET Core 2.0 web API in a Ubuntu 18 host, using Docker 18.05.0-ce (build f150324) and these services:

1.SQL Server database
2.MySql database
3.Mongo database

My docker compose file provides these services from their respective images, as reported below. I configure the database admin users with envronment variables, as required, and override connection strings in my API app using environment variables, too.

All the services start OK, MySql included; yet, my app throws an exception when trying to connect to MySql: Application startup exception: MySql.Data.MySqlClient.MySqlException (0x80004005): Host '172.19.0.5' is not allowed to connect to this MySQL server.

Could anyone suggest a solution? Here is my compose file:

version: '3.4'

services:
  # SQL Server at default port (1433, redirect to e.g. 1403 in a machine
  # with SQL Server installed).
  lexminmssql:
    image: microsoft/mssql-server-linux
    container_name: sqlserver
    environment:
      ACCEPT_EULA: Y
      SA_PASSWORD: "P4ss-W0rd!"
    ports:
      - 1433:1433
    networks:
      - lexminnetwork

  # MongoDB - at default port (27017, just stop your host MongoDB server if any,
  # or redirect it to another port).
  lexminmongo:
    image: mongo
    container_name: mongo
    ports:
      - 27017:27017
    networks:
      - lexminnetwork

  # MySql at default port (3306, redirect if required)
  # https://github.com/docker-library/docs/tree/master/mysql#mysql_database
  # https://docs.docker.com/samples/library/mysql/#environment-variables
  lexminmysql:
    image: mysql/mysql-server
    container_name: mysql
    environment:
      # the password that will be set for the MySQL root superuser account
      MYSQL_ROOT_PASSWORD: "password"
    ports:
      - 3306:3306
    networks:
      - lexminnetwork

  # Web API
  lexminapi:
    image: naftis/lexminapi
    ports:
      - 58942:58942
    depends_on:
      - lexminmssql:
        condition: service_healthy
      - lexminmongo:
        condition: service_healthy
      - lexminmysql:
        condition: service_healthy
    build:
      context: .
      dockerfile: LexminApi/Dockerfile
    environment:
      # for Windows use : as separator, for non Windows use __
      # (see https://github.com/aspnet/Configuration/issues/469)
      DATA__DEFAULTCONNECTION__CONNECTIONSTRING: "Server=sqlserver\\sqlexpress,1433;Database=lexmin;User Id=SA;Password=P4ss-W0rd!;MultipleActiveResultSets=true"
      SERILOG__CONNECTIONSTRING: "Server=sqlserver\\sqlexpress,1433;Database=lexmin;User Id=SA;Password=P4ss-W0rd!;MultipleActiveResultSets=true"
      LEX__CONNECTIONSTRING: "mongodb://mongo:27017/lex_catalog"
      ZAX__CONNECTIONSTRING: "Server=mysql;Database=zax_master;Uid=root;Pwd=password;SslMode=none"
      # TODO: locate BIN directories in Linux
      ENVIRONMENT__MONGODIRECTORY: ""
      ENVIRONMENT__MYSQLDIRECTORY: ""
      ENVIRONMENT__MYSQLDUMPUSER: "root"
      ENVIRONMENT__MYSQLDUMPPASSWORD: "password"
    networks:
      - lexminnetwork
    volumes:
      - ./zax-users.xml:/etc/lexmin/zax-users.xml

  # Web app
  # TODO

networks:
  lexminnetwork:
    driver: bridge