Add version property to the devcontainer.json spec (like devcontainer-feature.json and devcontainer-template.json)

I've been diving in deeper with devcontainers lately, publishing my own features and templates, etc. When publishing features and templates, the devcontainer cli seems to parse the version property out of the json file and apply it as the image tag (using `devcontainer feature/template publish`). It would be nice if the `devcontainer build --push ...` command could do the same. The version property could just be an optional property.

devcontainer.json ref:
https://containers.dev/implementors/json_reference/

devcontainer-feature.json ref:
https://containers.dev/implementors/features/

devcontainer-template.json ref:
https://containers.dev/implementors/templates/