Run onCreateCommand and updateContentCommand in prebuilt images (not pre-built Codespaces)

Codespaces has an option to bake `onCreateCommand` and `updateContentCommand` steps into the image in case they are time-consuming.

> If a user installs dependencies into a dev container via new features spec, they won't be able to reference those in the image's `Dockerfile` because features are applied after the initial build process. So the only way to customize the image becomes those aforementioned steps.

Providing a way to create an analog of Codespaces prebuilds would allow the publication of organization-wide fully ready-made images for local use.