nginx: [warn] could not build optimal proxy_headers_hash, you should increase either proxy_headers_hash_max_size: 512 or proxy_headers_hash_bucket_size: 64; ignoring proxy_headers_hash_bucket_size nginx: [emerg] bind() to 0.0.0.0:81 failed (98: Address already in use)

这个问题是由于Nginx配置中的proxy_headers_hash_bucket_size配置项值过小，或者proxy_headers_hash_max_size值过小，导致Nginx无法正确构建proxy_headers_hash。可以尝试增加proxy_headers_hash_max_size或proxy_headers_hash_bucket_size的值来解决该问题。同时，该错误还提示了端口81已经被占用，可以尝试更改Nginx监听的端口号。