Use vanilla versions of distros for build

Currently, we use the latest build of a particular distro by relying on upstream images, for example `ubuntu:20.04`. This is problematic, because `metapkg` generates dependencies on the system libraries with the `>=` condition, thus relying on the possibly updated library version as a minimum requirement. This may cause issues when the package is being installed on a vanilla (non-updated) distro instance.

The solution is to rely on base distro images that are specifically built on top of the initial release. In particular, this also requires disabling the "updates" repository which is enabled by default in most distros.