Assign custom class to post content images

As you are using bootstrap you can add img-fluid helper class to images like this.

function example_add_img_class( $class ) {
     
    return $class . ' img-fluid';
     
}
 
add_filter( 'get_image_tag_class', 'example_add_img_class' );

get_image_tag_class filter allows to add custom css class along side WordPress default classes. Read more about the filter here