Error on upload: "Impossible to create the root directory..."
I understand now within the context of you are using docker. Since docker creates a new file system separate from your computer's file system, you will want to run these commands on the docker instance. 

I do not use docker regularly, but I believe you can use docker attach while the droplet is running to access the droplet's file system.

You will want to set the parent folder to have higher privileges. Try this command: 

chmod 774 craftcms-assets


Privileges are composed of three numbers: self, group, and world. There are also three access levels: read, write, and execute. 7 will give access to read, write, and execute. You will want to use 7 for your own commands (self) and for craft's commands (group), but not for the general public (world).

Hope this helps!