Thanks! I just got it working:
I created a script in Ubuntu (in ~/bin) called godot_export.sh, and gave it permission to execute. This is the command:
~/GODOT/<godot executable> --export-release "Web Client" --headless --path <project path>
I set a shortcut through Ubuntu settings, to run the script, so I can export the project with Ctrl+E
Thanks for pointing me in the right direction!