Nginx proxy to S3

nginx-lua-s3.nginxconf

  location ~* ^/s3/(.*) {
    set $bucket           '<REPLACE WITH YOUR S3 BUCKET NAME>';
    set $aws_access       '<REPLACE WITH YOUR AWS ACCESS KEY>';
    set $aws_secret       '<REPLACE WITH YOUR AWS SECRET KEY>';
    set $url_full         "$1";
    set_by_lua $now       "return ngx.cookie_time(ngx.time())";
    set $string_to_sign   "$request_method\n\n\n\nx-amz-date:${now}\n/$bucket/$url_full";
    set_hmac_sha1          $aws_signature $aws_secret $string_to_sign;
    set_encode_base64      $aws_signature $aws_signature;

    resolver               172.31.0.2 valid=300s;
    resolver_timeout       10s;

    proxy_http_version     1.1;
    proxy_set_header       Host $bucket.s3.amazonaws.com;
    proxy_set_header       x-amz-date $now;
    proxy_set_header       Authorization "AWS $aws_access:$aws_signature";
    proxy_buffering        off;
    proxy_intercept_errors on;

    rewrite .* /$url_full break;

    proxy_pass             http://s3.amazonaws.com;
  }

I installed OpenResty, in order to set_by_lua, set_hmac_sha1 and others, and the problem I've run into is that the $url_full variable is not recognized by the rewrite command, so the URL is rewritten as just "/". This has the result of discarding the specific object URL and just trying to do a list on the S3 bucket, itself. Is there a way to get the rewrite directive to understand previously declared variables, rather than just the $1, $2, ... that are built into the rewrite directive?