Rework label handling (#387)

* be smarter about labels

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

kwmserver/Dockerfile

@@ -1,27 +1,25 @@
 ARG CODE_VERSION=1.1.1
 FROM kopano/kwmserverd:${CODE_VERSION}
 
-ARG VCS_REF
 ARG CODE_VERSION
-
 ENV CODE_VERSION="${CODE_VERSION}"
 
 LABEL maintainer=az@zok.xyz \
     org.label-schema.name="Kopano Kwmserver container" \
     org.label-schema.description="Container for running Kopano Kwmserver (WebRTC signalling server)" \
     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"
 
-USER root
-
 SHELL ["/bin/ash", "-eo", "pipefail", "-c"]
 
+USER root
 ENV DOCKERIZE_VERSION v0.11.0
 RUN wget -O - https://github.com/powerman/dockerize/releases/download/"$DOCKERIZE_VERSION"/dockerize-"$(uname -s)"-"$(uname -m)" | install /dev/stdin /bin/dockerize
+USER nobody
 
 COPY wrapper.sh /usr/local/bin
 
-USER nobody
+ARG VCS_REF
+LABEL org.label-schema.vcs-ref=$VCS_REF