TightVNC authentication failed, how to get encrypted password?
So you got few things wrong conceptually. The guide is absolutely fine. VNC has two parts VNC Server and VNC Viewer. When you installed VNC locally on your system you may have installed the server version which asks you for password. This password is for you system's VNC Server. With that a VNC Client name VNC viewer or something would have been installed also.
Now the docker image that you run hosts a VNC server on port 5901 and the password for connection is secret. So the only thing you need to do is Open VNC Viewer, Connect to :5901. When asked for password enter secret.
The dockerfile was shown to you for explaining how the author got the password and those commands have nothing to do with your system.