use DrupalnodeNodeInterface;
 use DrupalCoreEntityEntityInterface;
 use DrupalCoreEntityDisplayEntityViewDisplayInterface;

 function hook_entity_view_alter(&$build, EntityInterface $entity, EntityViewDisplayInterface $display){
   if($entity->getType() == 'relevant_type'){
    $build['#cache']['contexts'][] = 'headers:User-Agent';
   }

 }