config: adding labels
config: adding labels

the "Labels" field is not specified in the docker image spec, but is
present and used.
https://github.com/docker/docker/blob/master/image/spec/v1.2.md

This carries over that functionality

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>

vbatts/oci-image-spec,jonboulle/image-spec,opencontainers/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,opencontainers/image-spec,mattspencer-arm/image-spec,jonboulle/image-spec