老师您好，我在执行build命令的时候，就出现了这个错误，表示看不懂，请老师指导一下。

我的镜像源替换成了我个人的阿里云镜像仓库，pull其他镜像都没问题，挺快的。

[root@69iwentestserv dockerfiletest]# clear
[root@69iwentestserv dockerfiletest]# docker image build -t hello .
Sending build context to Docker daemon  3.072kB
Step 1/4 : FROM ubuntu:21.04
 ---> de6f83bfe0b6
Step 2/4 : RUN apt-get update &&     DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y python3.9 python3-pip python3.9-dev
 ---> Running in 4b1d4521eaf8
Get:1 http://security.ubuntu.com/ubuntu hirsute-security InRelease [110 kB]
Get:2 http://archive.ubuntu.com/ubuntu hirsute InRelease [269 kB]
Err:1 http://security.ubuntu.com/ubuntu hirsute-security InRelease
  gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
Err:2 http://archive.ubuntu.com/ubuntu hirsute InRelease
  gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
Get:3 http://archive.ubuntu.com/ubuntu hirsute-updates InRelease [115 kB]
Err:3 http://archive.ubuntu.com/ubuntu hirsute-updates InRelease
  gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
Get:4 http://archive.ubuntu.com/ubuntu hirsute-backports InRelease [101 kB]
Err:4 http://archive.ubuntu.com/ubuntu hirsute-backports InRelease
  gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
Reading package lists...
W: GPG error: http://security.ubuntu.com/ubuntu hirsute-security InRelease: gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
E: The repository 'http://security.ubuntu.com/ubuntu hirsute-security InRelease' is not signed.
W: GPG error: http://archive.ubuntu.com/ubuntu hirsute InRelease: gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
E: The repository 'http://archive.ubuntu.com/ubuntu hirsute InRelease' is not signed.
W: GPG error: http://archive.ubuntu.com/ubuntu hirsute-updates InRelease: gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
E: The repository 'http://archive.ubuntu.com/ubuntu hirsute-updates InRelease' is not signed.
W: GPG error: http://archive.ubuntu.com/ubuntu hirsute-backports InRelease: gpgv, gpgv2 or gpgv1 required for verification, but neither seems installed
E: The repository 'http://archive.ubuntu.com/ubuntu hirsute-backports InRelease' is not signed.
The command '/bin/sh -c apt-get update &&     DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y python3.9 python3-pip python3.9-dev' returned a non-zero code: 100
[root@69iwentestserv dockerfiletest]#