Add tear down test for rows created with `haveInDatabase`on a table without auto increment

[JesusTheHun]

1. Test rows created with haveInDatabase are correctly deleted during tear down if the table has no auto increment at all
2. Uniformise database connection by consistently using MYSQL_DSN instead of a combination of MYSQL_HOST and string template.
3. Swap MariaDB 10.5 for Mysql 5.7 in docker-compose.yml to be coherent with CI.
4. Create a non-root user in the Dockerfile.php81 to follow composer security guidelines.