Add metadata labels

merge-request-check/Dockerfile

 FROM python:alpine
 
-LABEL org.label-schema.name="Merge Requset Checker" \
+ARG BUILD_DATE
+ARG VCS_REF
+LABEL org.label-schema.build-date=$BUILD_DATE \
+      org.label-schema.name="Merge Requset Checker" \
       org.label-schema.description="Checks merge requests against ITS criteria" \
+      org.label-schema.vcs-ref=$VCS_REF \
       org.label-schema.vendor="University of Nebraska - Lincoln" \
       org.label-schema.version="0.1.0" \
       org.label-schema.schema-version="1.0" \