Rework label handling (#387)

* be smarter about labels

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

kdav/Dockerfile

@@ -7,7 +7,6 @@ RUN composer install
 
 FROM ${docker_repo}/kopano_base
 
-ARG VCS_REF
 ARG ADDITIONAL_KOPANO_PACKAGES=""
 ARG DOWNLOAD_COMMUNITY_PACKAGES=1
 ARG KOPANO_REPOSITORY_FLAGS="trusted=yes"
@@ -28,7 +27,6 @@ LABEL maintainer=az@zok.xyz \
     org.label-schema.name="Kopano kDAV container" \
     org.label-schema.description="Container for running Kopano kDAV" \
     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.schema-version="1.0"
 
@@ -87,3 +85,6 @@ ENV LANG en_US.UTF-8
 
 ENTRYPOINT ["/usr/bin/dumb-init", "--"]
 CMD [ "/kopano/start.sh" ]
+
+ARG VCS_REF
+LABEL org.label-schema.vcs-ref=$VCS_REF