When trying to build a Debian container, I encountered a bunch of errors like this upon running apt update:
W: GPG error: http://deb.debian.org/debian buster InRelease: At least one invalid signature was encountered.
E: The repository 'http://deb.debian.org/debian buster InRelease' is not signed.
The issue seems to be a lack of disk space. This StackOverflow answer was indeed correct: running docker image prune -f and docker container prune -f fixed the problem.