[feature] Enable passing --metadata-file arg to devcontainer build

Hi,

As we're using private registry, Jfrog Artifactory, we want to publish build info after devcontainer has been build, and pushed, so that JFrog can properly track the containers dependencies.

But currently there doesn't seem to be a straight forward way to make `devcontainer build` command export the metadata file, needed for input to JFrog CLI `jf rt build-docker-create --image-file` command.
It's possible using native `docker buildx build` (reference: https://docs.docker.com/engine/reference/commandline/buildx_build/#metadata-file)

I would like to be able to pass it like this: `devcontainer build --image-name ${IMAGE}:${TAG} --workspace-folder "src/${IMAGE}/" --metadata-file metadata.json`

If this is already possible with some temporary workarounds, I'd like to know how.