apt update throws signature error in Ubuntu 20.04 container on arm

The root cause of the problem is in libseccomp. The newer version fixes the problem, but it is not yet available in Debian's stable repos. There are two way to fix this problem:

Method 1

Start the container with --privileged. This bypasses docker's security measures, so it is not recommended. Or with --security-opt seccomp:unconfined as a little more secure. docker run -it --security-opt seccomp:unconfined ubuntu:latest

Method 2

Upgrade libseccomp manually on the host system. Dowload version from unstable repos (I tested with 2.4.3-1) here.

Install the new version:

sudo dpkg -i libseccomp2_2.4.3-1+b1_armhf.deb

Note: The above methods solved the problems for systems based on Raspbian. The error was also happening on a Ubuntu 20.04 aarch64 system, for which @NeonLines' answer was able to help.