You can save your drawing as a PNG file by clicking the "Save Canvas" button. Just be sure to provide the image with a file name. The image will save in your project's current working directory (most likely the same folder as your code).
If you want to programmatically save the canvas, you can use the save_canvas() function.
The function does launch the a dialog to enter the file's name.
If you want to use the save_canvas() function without launching the dialog each time, you must provide a file name to save the image as.