Repository is not signed in docker build

Apparently my root partition was full (maybe I've tried too many times to download packages through apt), and running sudo apt clean solved the issue

Since Docker API v1.25+ ( released: Nov 18, 2019 )

Running the command below fixed the problem for me:

docker system prune --force

The --force flag stands for noninteractive prune.

Additionally, you may want to give a try to the prune volume commands:

docker volume prune --force

fixed by

docker image prune -f

looks like docker has a limit on maximum apt cache size on the host system

For Raspbian, upgrade libseccomp manually on the host system by using:

curl http://ftp.us.debian.org/debian/pool/main/libs/libseccomp/libseccomp2_2.5.1-1_armhf.deb --output libseccomp2_2.5.1-1_armhf.deb
sudo dpkg -i libseccomp2_2.5.1-1_armhf.deb

This resolved my issue.

Original post is here.