This commit installs mysqldump via the package mariadb-client to make
use of `php artisan backup:run`. Without that package the command fails
with the following error:
```
$ docker-compose exec app php artisan backup:run
Starting backup...
Dumping database pixelfed...
Backup failed because The dump process failed with exitcode 127 : Command not found : sh: 1: mysqldump: not found
```