Update documentation for non-root container users

Following discussions in !2309 (merged), we need to update our documentation to instruct users how to set up their user to control Funkwhale containers without root. This is currently not supported, but work is ongoing in !2250 (closed) to set up the containers to use a dedicated user that can map to a server user.

The documentation currently instructs users to set up a user called funkwhale and add them to the docker group per Docker's post-install instructions. This grants the user access to the Unix socket that Docker runs on. With the new user created inside the containers, the funkwhale user should be able to control the containers without requiring low-level access.