http {
    ...

    map $request_method $is_purge {                                                             
        default   0;
        PURGE     1;
    }

    proxy_cache_path /path/to/cache levels=1:2 keys_zone=my_cache:10m;
    proxy_cache_key $scheme$proxy_host$request_uri;

    server {
        ...

        location / {
            if ($is_purge) {
                set $my_cache_key $scheme$proxy_host$request_uri;

                content_by_lua_block {
                    local exitStatus = os.execute("/usr/local/bin/nginx-cache-purge /path/to/cache 1:2 "..ngx.var.my_cache_key)
                     
                    if exitStatus == 0 then
                        ngx.exit(ngx.HTTP_OK)
                    else
                        ngx.exit(ngx.HTTP_BAD_REQUEST)
                    end
                } 
            }

            proxy_pass upstream;
            include proxy_params;
        }
    }
}