Improve documentation of dev container image prebuild and update templates

[craiglpeters]

Many developers want to understand how to enable the reuse of an existing container image each time a session is started, as opposed to rebuilding the container from scratch so that:

• Create a codified build event for deterministic environment definition
• Speed environment creation
• In the Codespaces Prebuilds case, work around the limitation that Codespaces do not follow a fork by

We documentation prebuilding dev container images, but none of our templates illustrate good practices for this, and the documentation is a but buried.

I propose that we create a guide that describes how to decide if you need to prebuild, walk through the mechanics on GitHub with Actions and GHCR, and provides a few good examples.
It would also help if we update the templates where a prebuilt image would help to illustrate the pattern.

[Clockwork-Muse]

.... prebuilding should probably call out whether the intent is to pre-build the current devcontainer, or a base image for children devcontainers. This is important because often what people want to do (for non-codespaces environments) is pre-build the "current" devcontainer, but there currently isn't a completely supported way to do that, in that;

• You can build and push an image.
• You can reference this in a cacheFrom line.
• You can't reference this in the image tag (although there was a proposal allowing this when also containing a build section, although I can't find it at the moment), although that's probably what people want/anticipate (and attempting to do so requires features to be idempotent, which isn't a guarantee in the wider ecosystem).
• This ends up meaning you're at the mercy of the caching system, which can break/bust at unexpected moments, especially around feature ordering.
• Or you need a separate build-vs-use devcontainer.json, which is undersirable.

[meastp]

I'm kind of struggling with this right now devcontainers/template-starter#26 and pre-building and uploading the (base-)template is the solution, it seems.

More elaborate documentation would be welcome (using devcontainer-ci?), but also ideally an extra github action in the template-starter repo to build and (all) upload the template(s) in the repo...