devcontainers
Updates "outdated" command to display stale image versions
Recently, we integrated Dependabot and Dev Containers where we get PRs to update versions for stale Features.
Similarly, for the devcontainer/images, we have been worried of bumping a major version of an image because folks could be left behind on stale images (which doesn't receive security patches). Currently, we don't have a good way of alerting the community when a new image version becomes available (besides putting up notices and announcing in the community channels). Out of public repos, 2600 dev containers are using the stale mcr.microsoft.com/devcontainers/python image (older major version which ww don’t support).
Hence, this PR updates the outdated command to include the stale image versions. It looks for the .devcontainer.json file, and looks for corresponding images in .devcontainer.json, Dockerfiles or Compose files (based on the dev container structure)
The outdated command currently supports only the images hosted from https://github.com/devcontainers/images. Reasoning: I looked at some popular images (eg. python, node, go, php etc), however, they don’t use semantic versioning. Hence, doesn’t make sense to implement for them.
