These published images are low-level OS support layers, on top of which you can install your developers' tools and configure their default environment.
Example of how to customize the Ubuntu 20.04 (Focal Fossa) image:
# syntax=docker/dockerfile:1
FROM ghcr.io/itopia-inc/spaces-base-images/spaces-ubuntu-base:focal
# Install your developers' tools
# Configure your developers' default environmentQuestions? Requests? Please let us know on GitHub!