What is the cause of the error "Gtk: cannot open display: :0" when running Chromium in Docker?

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The error "Gtk: cannot open display: :0" occurs when Chromium is trying to open a graphical user interface, but the Docker container does not have access to the X Windows display. This could be because the container is running on a headless server, or the DISPLAY environment variable is not set correctly in the container. To fix this error, you can either use a different headless browser or make sure the Docker container has access to the X Windows display by setting the DISPLAY environment variable to the correct value.