Setting the INPUT_ADD_GIT_LABELS env var to true adds labels as per the
opencontainers standard: https://github.com/opencontainers/image-spec/blob/master/annotations.md
The 3 labels supported are:
- org.opencontainers.image.created
- org.opencontainers.image.source
- org.opencontainers.image.revision
Signed-off-by: Nick Adcock <nick.adcock@docker.com>