Rework label handling (#387)

* be smarter about labels

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

php/Dockerfile

@@ -1,7 +1,6 @@
 ARG docker_repo=zokradonh
 FROM ${docker_repo}/kopano_base
 
-ARG VCS_REF
 ARG ADDITIONAL_KOPANO_PACKAGES=""
 ARG DOWNLOAD_COMMUNITY_PACKAGES=1
 ARG KOPANO_REPOSITORY_FLAGS="trusted=yes"
@@ -22,7 +21,6 @@ LABEL maintainer=az@zok.xyz \
     org.label-schema.name="Kopano php container" \
     org.label-schema.description="Base container for running php based applications based on 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"
@@ -55,3 +53,6 @@ EXPOSE 9080/tcp
 
 COPY start-helper.sh /kopano/start-helper.sh
 COPY kweb.cfg /etc/kweb.cfg
+
+ARG VCS_REF
+LABEL org.label-schema.vcs-ref=$VCS_REF