Ubuntu 18.04 - get Ubuntu dock to only display the windows from the current screen (monitor)
Solution 1:
Launch Terminal and run
gsettings set org.gnome.shell.extensions.dash-to-dock isolate-monitors true
Solution 2:
You could also achieve the same by using the dconf Editor application (that can be found in Ubuntu store).
Just go to /org/gnome/shell/extensions/dash-to-dock
and search for isolate-monitors
and toggle its value to make the change.