The dockerfile uses a container with debian 10 which is EOL and thus mysql will not install anymore. i checked it by running cat /etc/os-release while running the docker container with php
out put of docker ps -a :
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
916ecb22b433 nginx "/docker-entrypoint.…" 3 hours ago Up 2 hours 0.0.0.0:80->80/tcp, [::]:80->80/tcp dotproject-devel-nginx-1
89056f73c700 php:7.4.0-fpm "docker-php-entrypoi…" 3 hours ago Up 2 hours 9000/tcp dotproject-devel-phpfpm-1
20a89eb7c071 mariadb "docker-entrypoint.s…" 3 hours ago Up 2 hours 3306/tcp dotproject-devel-mariadb-1
vboxuser@debis:~/Downloads/dotProject-devel$
i picked the one in the middle