`--label` option for `devcontainer build`

[eitsupi]

Maybe related to devcontainers/spec#18

Currently the only way to set labels seems to be to write to Dockerfiles or compose files, but if we store images in the registry, we may want to annotate the labels.
https://github.com/opencontainers/image-spec/blob/main/annotations.md

[danielloader]

This has come up for me today, glad to see a tracking issue, shame it's not been looked at yet.

[samruddhikhandale]

Looks like this feature request slipped through the gaps, however we saw the latest #837 request

Have #846 open which attempts to support --label