problema accesso a phpmyadmin: mysqli_real_connect(): php_network_getaddresses: getaddrinfo failed

[astros3]

+ cat docker-compose.yml
version: '3'
services:
    phpmyadmin:
        image: phpmyadmin/phpmyadmin
        ports:
            - 8181:80
        links:
            - db
    db:
        image: mysql:5.7

+ docker-compose up --build -d
Recreating phpmyadmin-docker_db_1 ... done
Recreating phpmyadmin-docker_phpmyadmin_1 ... done

Apro il browser e mi collego a http://localhost:8181
Digito root (come nome e password) e premo esegui.
Ottengo questo risultato:
https://i.ibb.co/x7CV3Bc/Schermata-d...4-12-04-09.png
Perchè?