location / {
    ...
    proxy_set_header Accept-Encoding "gzip";
    ...
    proxy_pass ...;
}

$realpath_root：

an absolute pathname corresponding to the root or alias directive’s value for the current request, with all symbolic links resolved to real paths

如果是符号链接，$realpath_root指向符号链接的真实路径

$document_root：

root or alias directive’s value for the current request

$document_root 是root或alias指定的值