Ghost Docker SMTP setup

I created a ghost instance in my vps with the official docker compose file of the ghost cms

and I modified it to use a mailgun SMTP account as follows

version: '3.1'
services:
  mariadb:
    image: 'docker.io/bitnami/mariadb:10.3-debian-10'
    environment:
      - ALLOW_EMPTY_PASSWORD=yes
      - MARIADB_USER=bn_ghost
      - MARIADB_DATABASE=bitnami_ghost
    volumes:
      - 'mariadb_data:/bitnami'
  ghost:
    image: 'ghost:3-alpine'
    environment:
      MARIADB_HOST: mariadb
      MARIADB_PORT_NUMBER: 3306
      GHOST_DATABASE_USER: bn_ghost
      GHOST_DATABASE_NAME: bitnami_ghost
      GHOST_HOST: localhost
      mail__transport: SMTP
      mail__options__service: Mailgun
      mail__auth__user: ${MY_MAIL_USER}
      mail__auth__pass: ${MY_MAIL_PASS}
      mail__from: ${MY_FROM_ADDRESS}
    ports:
      - '80:2368'
    volumes:
      - 'ghost_data:/bitnami'
    depends_on:
      - mariadb
volumes:
  mariadb_data:
    driver: local
  ghost_data:
    driver: local

but when I try to invite authors to the site it gives me following error

Failed to send 1 invitation: dulara@thinksmart.lk. Please check your email configuration, see https://ghost.org/docs/concepts/config/#mail for instructions

I am certain that my SMTP credentials are correct.

I logged in to ghost containers bash shell and checked its files there. it's mail section is empty

I still can't find what is my mistake. I am not sure about the variable names. but I took them from the official documentation.