ssh-tunnel: ssh server to allow (reverse) ssh tunnels

To avoid need for privilged container:
- listen on port 2222 (and not 22)
- add user 'visitor' to avoid login as root

Notes:
- expects ssh pubkey (rather than password)
- allow client-speicfied GatewayPorts (reverse tunnel)

Example how to run with two ports for reverse tunnel:

  AUTH=authorized_keys	# path to authorized_keys file
  PORT=22222			# external port for the container

  TUN1=20001
  TUN2=20002

  docker run -d -v $AUTH:/home/visitor/.ssh/authorized_keys:ro \
          -p $PORT:2222 -p $TUN1:$TUN1 -p $$TUN2:$TUN2 -t sshd-tunnel