How to overwrite mysql sql_mode parameter in docker-compose

I wrote docker compose file

    version: '3.9'
services:
  db:
    image: mysql:5.7
    restart: always
    environment:
      - MYSQL_ROOT_PASSWORD=root
    ports:
      - 3306:3306
    volumes:
      - mysql_db:/var/lib/mysql
      - ./my.conf:/etc/mysql/conf.d/my.cnf

volumes:
  mysql_db:

And my.conf

[mysqld] 
sql-mode="TRADITIONAL,ALLOW_INVALID_DATES"

But after restart container parameters in my.conf file was added to default sql_mode

    mysql> show variables like "sql_mode";
+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Variable_name | Value                                                                                                                                                                    |
+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| sql_mode      | STRICT_TRANS_TABLES,STRICT_ALL_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ALLOW_INVALID_DATES,ERROR_FOR_DIVISION_BY_ZERO,TRADITIONAL,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION |
+---------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0,00 sec)

How to overwrite default parameters and left only present in my.conf ???