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While attempting to rebuild a docker container FROM phusion/baseimage on my old Raspberry Pi 3 Debian Buster host I ran into the following error:

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.

Didn’t make sense at first, but a StackOverflow answer pointed me in a direction to a bug in libseccomp2 – a package I’ve never actually heard of. Short story long, the problem is fixed, and has been fixed for years, but because debian stable is, well, stable, the fix didn’t land in the buster repos.

However, installing libseccomp2 2.5.1 from buster-backports on the host didn’t actually resolve the issue.

Another post on the moby github issue suggested that docker > 19 has worked around the issue internally. So I uninstalled all of my previous docker images and reinitialized via the bootstrap script and was able to successfully run apt-get update on focus-based containers!