Linux fd 5 是什么文件描述符

用 strace docker build.....的过程中，发现在 npm install --registry=xxx.com 那一步，有些问题：

read(5, 0xc00048d000, 4096)             = -1 EAGAIN (Resource temporarily unavailable)
epoll_pwait(4, {{EPOLLOUT, {u32=3554582280, u64=139937884053256}}}, 128, 0, NULL) = 1
epoll_pwait(4, {{EPOLLIN|EPOLLOUT, {u32=3554582280, u64=139937884053256}}}, 128, -1, NULL) = 1
futex(0x7f45d8a0b0d0, FUTEX_WAKE_PRIVATE, 1) = 1
read(5, "33\r\n{\"stream\":\" ---\\u003e Runnin"..., 4096) = 57
write(1, " ---> Running in 161d1d6bb0fc\n", 30 ---> Running in 161d1d6bb0fc
) = 30
read(5, 0xc00048d000, 4096)             = -1 EAGAIN (Resource temporarily unavailable)
epoll_pwait(4, {}, 128, 0, NULL)        = 0
epoll_pwait(4, // [在这卡住不动了] 

有两个不太理解的地方，请老哥指点：

• fd 4 5 ，分别表示什么，google 没人说清楚，都是说 1 2 3 的
• 我 ping/curl registry 地址是没问题的，在另一台设备 npm install --registry 也是正常的，现在我该如何排查🤏

补充

根据 linux read 的描述 ssize_t read(int fd, void *buf, size_t count);

read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.