How to automatically add custom attribute in IMG tag in CKEditor?

Question

Django

Good day.
I need to automatically add data-fancybox to the img tag. Tell me how to do it? I understand that it is necessary to write the code in config.js , but I don’t quite understand which one.

Here is a code example for what the class would add and it works.

CKEDITOR.on( 'instanceReady', function( evt ) {
  evt.editor.dataProcessor.htmlFilter.addRules( {
    elements: {
    img: function(el) {
      el.addClass('img-responsive');
    
    }
    }
  });
  });