pe0m What would you suggest smajor do?
OP is running docker. Basic (ideally more than basic) knowledge of unix permissions and process/file ownership is required to do so. There is no FTP involved. What I suggest OP should do is exactly what the error message says: check the permissions.
This means figuring out how the used container is running PHP, how the volume is mounted and what permissions need to be set on the host system so that the container process can access and write the files in the volume.
All of this is way beyond the scope of this forum. The basic knowledge of how to run a PHP web application is needed to run DokuWiki. That required knowledge differs depending on where and how you host and we give hints on what to look out for in these different environments, but we can't teach you everything.
And yes, I'm a bit salty about people who do not have the knowledge to run a single Linux system, now "simply" running Docker without realizing that they now are running two or more Linux systems they don't understand.