Mac docker构建镜像apt-get update报GPG签名错误解决

我正在使用python37作为基础镜像制作项目镜像，但构建过程中执行apt-get update时出现GPG签名错误

本地操作系统为Mac OS，docker 环境为desktop for docker ：

Docker version 20.10.7, build f0df350

错误详情：

Err:1 http://ports.ubuntu.com/ubuntu-ports focal InRelease
  At least one invalid signature was encountered.
Get:4 http://ports.ubuntu.com/ubuntu-ports focal-security InRelease [107 kB]
Err:2 http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease
  At least one invalid signature was encountered.
Err:3 http://ports.ubuntu.com/ubuntu-ports focal-backports InRelease
  At least one invalid signature was encountered.
Err:4 http://ports.ubuntu.com/ubuntu-ports focal-security InRelease
  At least one invalid signature was encountered.
Reading package lists... Done
W: GPG error: http://ports.ubuntu.com/ubuntu-ports focal InRelease: At least one invalid signature was encountered.
E: The repository 'http://ports.ubuntu.com/ubuntu-ports focal InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease: At least one invalid signature was encountered.
E: The repository 'http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.

解决方案：

docker image prune -a

会清理一些镜像，然后再进行构建，就没有报错了

参考文章：stackoverflow