Rework label handling (#387)

* be smarter about labels

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

utils/Dockerfile

@@ -5,7 +5,6 @@ LABEL maintainer=az@zok.xyz \
     org.label-schema.name="Kopano utils container" \
     org.label-schema.description="Container that bundles various cli tools from Kopano Groupware Core" \
     org.label-schema.url="https://kopano.io" \
-    org.label-schema.vcs-ref=$VCS_REF \
     org.label-schema.vcs-url="https://github.com/zokradonh/kopano-docker" \
     org.label-schema.version=$KOPANO_CORE_VERSION \
     org.label-schema.schema-version="1.0"
@@ -27,3 +26,6 @@ RUN apt-get update && apt-get install --no-install-recommends -y \
     && rm -rf /var/lib/apt/lists/*
 
 CMD [ "/bin/bash" ]
+
+ARG VCS_REF
+LABEL org.label-schema.vcs-ref=$VCS_REF