Rework label handling (#387)

* be smarter about labels

inspired by https://github.com/Peter-SAARLAND/zero

ssl/Dockerfile

@@ -1,7 +1,5 @@
 FROM alpine:3.11
 
-ARG VCS_REF
-
 ENV CODE_VERSION=1.2.0 \
     PKI_ROOT=/kopano/easypki \
     PKI_ORGANIZATION="Internal Kopano System" \
@@ -11,7 +9,6 @@ LABEL maintainer=az@zok.xyz \
     org.label-schema.name="Kopano SSL container" \
     org.label-schema.description="Helper Container for carrying out ssl related activities for the Kopano stack." \
     org.label-schema.url="https://kopano.io" \
-    org.label-schema.vcs-ref=$VCS_REF \
     org.label-schema.vcs-url="https://github.com/zokradonh/kopano-docker" \
     org.label-schema.version=$CODE_VERSION \
     org.label-schema.schema-version="1.0"
@@ -28,3 +25,6 @@ COPY start.sh /start.sh
 RUN chmod a+x /start.sh
 
 CMD ["/start.sh"]
+
+ARG VCS_REF
+LABEL org.label-schema.vcs-ref=$VCS_REF