Arbitrary labels could be added as `make` parameter `LABEL=`, which is used in
att-comdev/cicd Jenkins pipelines.
Sample parameter:
'LABEL=org.label-schema.vcs-url=${GERRIT_CHANGE_URL} \
--label org.label-schema.base-image=${base_sha256}'
Sample usage:
See Jenkinsfile files under images/ directory in att-comdev/cicd repo.
In addition to that, if `COMMIT` variable is undefined when invoking `make`, we
use result of `git rev-parse HEAD` command, which should output latest git
commit ID.
[0] https://github.com/att-comdev/cicd
Change-Id: I9193ca03cb3970e3a3e1cb64ff643a4d24543636
2c09054c4b