I am using Python 37 as the basic image to create the project image, but there is a GPG signature error when executing apt get update during the construction process

The local operating system is Mac OS, and the docker environment is desktop for docker:

Docker version 20.10.7, build f0df350

Error details:

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.

Solution:

docker image prune -a

Some images will be cleaned up and then built. There will be no errors