Incorrect Exit node environment variables

[RoboMagus]

While trying to configure the docker mod for creating an exit node I came accross an inconsistency in the use of environment variables causing the setup of exit node to be more problematic than it should be:

if [ -v TAILSCALE_BE_EXIT_NODE ]; then
    echo '[!] acting as an exit node, you may need to approve this in the admin console'
    FLAGS="${FLAGS} --advertise-exit-node=${TS_BE_EXIT_NODE}"
fi

As well as it not being mentioned in the documentation.