Windows have no buttons(maximizing,close...) in docker container after connecting via VNC Viewever

Question

I installed the vnc server in a running docker container using the following commands.

sudo apt-get install -y x11vnc

Then I run the vnc server inside docker container.

export DISPLAY=:20 Xvfb :20 -screen 0 1366x768x16 & x11vnc -passwd TestVNC -display :20 -N -forever &

Then I connected via VNC Viewever.

After starting a window inside container, the window has no buttons on it (As you can see from the picture)

What can be solution for this issue ?

Thanks

Here is the example image