The repository ... is not signed error in docker build apt-get update

Every time I encountered it, I forgot the cause and wasted my time, so make a note of it so that I can remember it.

Event

When I try to build an Ubuntu 20.04 image with Docker for Mac and do the usual ʻapt-get updatein the Dockerfile, the followingThe repository ... is not signed.` error may occur. There is.

Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [107 kB]
Get:2 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
Err:1 http://security.ubuntu.com/ubuntu focal-security InRelease
  At least one invalid signature was encountered.
Err:2 http://archive.ubuntu.com/ubuntu focal InRelease
  At least one invalid signature was encountered.
Get:3 http://archive.ubuntu.com/ubuntu focal-updates InRelease [111 kB]
Err:3 http://archive.ubuntu.com/ubuntu focal-updates InRelease
  At least one invalid signature was encountered.
Get:4 http://archive.ubuntu.com/ubuntu focal-backports InRelease [98.3 kB]
Err:4 http://archive.ubuntu.com/ubuntu focal-backports InRelease
  At least one invalid signature was encountered.
Reading package lists...
W: GPG error: http://security.ubuntu.com/ubuntu focal-security InRelease: At least one invalid signature was encountered.
E: The repository 'http://security.ubuntu.com/ubuntu focal-security InRelease' is not signed.
W: GPG error: http://archive.ubuntu.com/ubuntu focal InRelease: At least one invalid signature was encountered.
E: The repository 'http://archive.ubuntu.com/ubuntu focal InRelease' is not signed.
W: GPG error: http://archive.ubuntu.com/ubuntu focal-updates InRelease: At least one invalid signature was encountered.
E: The repository 'http://archive.ubuntu.com/ubuntu focal-updates InRelease' is not signed.
W: GPG error: http://archive.ubuntu.com/ubuntu focal-backports InRelease: At least one invalid signature was encountered.
E: The repository 'http://archive.ubuntu.com/ubuntu focal-backports InRelease' is not signed.

I look at the message and think "Is it a signature problem?", But every time I waste time saying "No, I know if the Ubuntu version is earlier, but the image tag is the latest and it's strange to wake up ..." Was ...

Cause

The conclusion of my case was that ** the root partition usage was 100% of the Docker disk image size **.

Every time I notice it, I feel weak ...

root@c761181b1d64:/# df -h
Filesystem      Size  Used Avail Use% Mounted on
overlay          30G   28G     0 100% /
...

This can be avoided by increasing the Disk image size of Docker for Mac, but it was a machine that could not afford to increase it ...

I avoided it safely by freeing up space with docker image prune which deletes all unused images and docker container prune which deletes all stopped containers.

As long as you continue to use a machine that does not have enough disk space, you will see an error again. The next time I encounter it, I'll solve it smoothly ...

Reference link

Repository is not signed in docker build | stack overflow