The configuration below works just fine on my local machine:

My docker-compose.yml file:

networks:
    mynet:
        name: mynet
        driver: bridge
services:
    mydb:
        container_name: mydb
        image: mysql:8.4.2
        hostname: mydb
        restart: unless-stopped
        environment:
            MYSQL_ROOT_PASSWORD: ${ROOT_PASSWORD}
            MYSQL_DATABASE: default_db
        networks:
            - mynet
        volumes:
            - ${DATA_VOLUME}:/var/lib/mysql
        ports:
            - ${EXPOSE_PORT}:3306

    myadmin:
        container_name: myadmin
        image: phpmyadmin/phpmyadmin:5.2.1
        hostname: myadmin
        depends_on:
            - mydb
        restart: unless-stopped
        environment:
            PMA_HOST: mydb
            # PMA_PORT: 3306 - I tried with this parameter too
            # PMA_ARBITRARY: 1 - I tried with this parameter too
            # MYSQL_USERNAME: "root" - I tried with this parameter too
            MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
        networks:
            - mynet
        ports:
            - ${PMA_EXPOSE_PORT}:80

And here is the .env file:

ROOT_PASSWORD=mystrongpwd
DATA_VOLUME=/path/to/my/colume
EXPOSE_PORT=3333
PMA_EXPOSE_PORT=4444

But when I’m trying to login to myadmin, I get this error:

mysqli::real_connect(): (HY000/1045): Access denied for user
‘root’@’172.26.0.12’ (using password: YES)