Potential for devcontainer improvements with feature layer

[wdconinc]

(I'm mostly posting this here as part of the follow-up to the HSF Training tutorial in November 2023, based on post-evaluation and review of our internal notes on the slack channel. FYI @klieret )

What would make loading the container faster is to create the root install as a feature layer as explained at https://github.com/devcontainers/feature-starter. That's something that is going to take a bit of work (in part because it requires the root-project org maintainer to allow access to OCI layers in ghcr.io). That would prepopulate ghcr.io with a single layer that adds root to the standard codespaces image. This prepopulated layer could be created and published in CI.