Strategy to pass `--label` to build command

[consideRatio]

I'd like to use chartpress together with the docker/metadata-action that provides standardized image labels and values - to set a few labels on built images. When using ghcr.io, you often want to couple a built image with a GitHub repository, and the OCI standardized labels can help do that.

The docker/metadata-action can emit the following labels for example.

        "org.opencontainers.image.title": "Hello-World",
        "org.opencontainers.image.description": "This your first repo!",
        "org.opencontainers.image.url": "https://github.com/octocat/Hello-World",
        "org.opencontainers.image.source": "https://github.com/octocat/Hello-World",
        "org.opencontainers.image.version": "1.2.3",
        "org.opencontainers.image.created": "2020-01-10T00:30:00.000Z",
        "org.opencontainers.image.revision": "90dd6032fac8bda1b6c4436a2e65de27961ed071",
        "org.opencontainers.image.licenses": "MIT"

#142 would add support for declaring extraBuildCommandOptions in chartpress.yaml to set labels via --labels flag, but updating chartpress.yaml dynamically wouldn't be a good idea as that is a hint for chartpress to rebuild all images etc as well. So, either there is something fixed in chartpress.yaml that makes use of the expanded variables like...

            expansion_namespace = {
                "LAST_COMMIT": _get_latest_commit_tagged_or_modifying_paths(
                    *all_image_paths, echo=False
                ),
                "TAG": image_tag,
            }

Brainstorming

1. chartpress to automatically pass --labels?
2. Updating Dockerfile with --build-args to set labels?
3. Updating chartpress.yaml with Allow extra options to be passed to docker build #142 and namespace expansion of LAST_COMMIT etc to set for example org.opencontainers.image.revision and org.opencontainers.image.version to reference TAG?

I'm leaning towards the idea of bypassing use of docker/metadata-action and relying entirely on namespace expansion and the new #142 feature, where we could perhaps also add a date expansion to support setting org.opencontainers.image.created.