If you only want to add an extra class, then you should use wp_get_attachment_image. It has few extra params, and the last one is used for setting class names.
Sample usage:
<?php echo wp_get_attachment_image( get_the_ID(), 'thumbnail', "", ["class" => "my-custom-class"] ); ?>
The main advantage of this aproach is that you will also get the whole srcset attributes for free.
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