KBExit thanks for the reply, yes the "/var/www/html/."folder is not empty but, as I said it wasn't the folder I run the command in (I don't use the "/var/www/html/" of my system) I have all my project in the /docker-dev/ folder.
And no I'm not installing inside of a docker image, my docker-dev folder is a folder for my projects including my docker-compose, dockerfiles etc. and my working www (which is mounted as a volume in my php container). I already have my flarum (v1.8) in that www among other project, running perfectly.
I'm running the command in my local terminal, not in my docker container.
I might be mistaken but the .means "in the current folder" so why is the command trying ot install in /var/www/html/." when I'm running it in a different folder?