Repository is not signed in docker build

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

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

---

In addition, the following commands should help clean up space:

docker system df # which can show disk usage and size of 'Build Cache'
docker image prune # add -f or --force to not prompt for confirmation
docker container prune # add -f or --force to not prompt for confirmation