Rework label handling (#387)

* be smarter about labels

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

python/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 Python container" \
     org.label-schema.description="Container for running python based applications for 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"
@@ -37,4 +35,7 @@ RUN \
     apt-get update && apt-get install -y --no-install-recommends \
         python3-kopano \
         ${ADDITIONAL_KOPANO_PACKAGES} \
-    && rm -rf /var/cache/apt /var/lib/apt/lists
+    && rm -rf /var/cache/apt /var/lib/apt/lists
+
+ARG VCS_REF
+LABEL org.label-schema.vcs-ref=$VCS_REF