【結論】
~~proxy_set_header : proxy_pass => currentレベル合わせる~~
=> 別に合わせなくても良かったという・・・なんでだ？

【結果】

nginx.conf


          proxy_set_header X-Real-IP              $remote_addr;

          location / {
              proxy_set_header Host                   $host;
              proxy_set_header X-Forwarded-Host       $host;
              proxy_set_header X-Forwarded-Server     $host;
              proxy_set_header X-Forwarded-For        $proxy_add_x_forwarded_for;
 
              proxy_pass http://localhost:8181;
          }

["HTTP_HOST"]=>
string(9) "localhost"
["HTTP_X_FORWARDED_HOST"]=>
string(9) "localhost"
["HTTP_X_FORWARDED_SERVER"]=>
string(9) "localhost"
["HTTP_X_FORWARDED_FOR"]=>
string(9) "127.0.0.1"
=> X-Real-IP無し

【参考】
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