Adding labels to containers build by devcontainer cli that are based on a Dockerfile (instead of an image) does not work

A few months ago, the `--label` property to `build` was added that adds labels (https://github.com/devcontainers/cli/pull/846). Unfortunately it seems that this only works when a devcontainer is build that is based directly from an `image`. If it is based from a `Dockerfile`, the labels from `--label` are not added.
I added a test in a PR that shows the problem, see here: https://github.com/devcontainers/cli/pull/929