For quite some time now I’ve been struggling with WordPress adding an additional 10px of padding to images that are uploaded via the content editor. After a small amount of digging, I’ve discovered the following override:

function remove_caption_padding( $width ) {
                  return $width - 10;
                add_filter( 'img_caption_shortcode_width', 'remove_caption_padding' );

Add this to your functions.php file and instantly remove the 10px padding.