introduce VNC_PASSWORDLESS

[felixhummel]

Hi!

This introduces the environment variable VNC_PASSWORDLESS to disable the VNC password.

Our use-case is simple: we have a reverse proxy in place that does authentication and authorization already, so having to supply a password (or append it to the URL) is just bad UX for us.

It would be great if you accepted this.

I only tested this locally with ubuntu.xfce, e.g.

docker build -t vnc -f Dockerfile.ubuntu.xfce.vnc .
docker run -e VNC_PASSWORDLESS=true -p 5901:5901 -p 6901:6901 vnc

Thanks for creating this container! :)

Best, Felix