The optimized devcontainer workflow downloads an image from the
GitHub container registry. Now that we've made that image, which is
built in CI, public, it would help to have the correct org name.
This commit adds a GitHub Workflow to prebuild the dev container file
present at .devcontainer/devcontainer.json. This prebuilt image is
pushed to GitHub Container Registry (ghcr).
An additional devcontainer.json is added consuming that image.
This dev container can be selected in editors that process dev
containers (ie GitHub Codespaces), to speed up time to editor.