Update doc for Docker in the rootless mode

[plu-project]

This pull request adds solution for users using Docker in the rootless mode. For users using Docker in the rootless mode, the existing Docker installation and the example command prevents FastSurfer from running.

Original Problems:

1. There is an additional need to install the NVIDIA container toolkit and configure it for Docker in the rootless mode. Otherwise, this error message raises: "docker: Error response from daemon: could not select device driver "" with capabilities: [[gpu]]".
2. In the example command, the --user flag mapping with the group- and user-id leads to the user in the container as "I have no name!" and therefore the user in the container does not have the write-access to the mounted output directory, even though the output directory in the host system has given permission to everyone.

To be noted:
The "--user 0" in the example command in the pull request is needed for Docker in the rootless mode. Without this, Docker complains "Error response from daemon:unable to find user nonroot".

[dkuegler]

Thank you for your contribution!