What Permissions Do mergefs, Docker and SMB Need?

I think that your OS is creating files / folders with root user and the containers are creating files / folders with appuser.

there is one simple solution: change ownership of the files from root to appuser

sudo chown -R appuser <path_to_your_folder>

Quote from darkarn

I am just not sure how to ensure that the permissions will be correct as it seems to me that mergefs sets all of the data to be 2777, root:users while the docker containers are 2777, appuser:users; whenever the docker containers do something to the data, the perms get overwritten and I am not sure if this is intended

The permissions 2777 numbers mean the following:

The 777 are the permissions for owner, group and others with each digit apply individually (first 7 is owner, second is group, third is others)

7 means that all permissions (read/write/execute) are allowed for that entity.

The 2 is the setgid bit that assists in sharing functions between groups and for the most part can be ignored for the purpose of this thread.

If you have 777, There are no restrictions imposed on any user/group so there should be not negative issues with files and directories being manipulated by docker/samba/mergerfs and so on, and you should not have to concern yourself with this in the context you asked.

If, however, you are trying to be security consious and restrict access, then the 777 might be too "open"

This lik will let you see what those numbers mean.

Chmod 2777