What’s the benefit of using the -onbuild vs. not using it in the CLI?
All images with -onbuild in their title will install the packages in packages.txt, python packages in requirements.txt when you run astro dev start as well as a few other commands (more info here).
Unless you explicitly have a reason against it (e.g. installing packages from an internal repository or private github repo), you should probably stick to using the onbuild images.