Rework label handling (#387)

* be smarter about labels

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

playground/Dockerfile

@@ -1,4 +1,4 @@
-from alpine:3.11 as builder
+FROM alpine:3.11 as builder
 
 RUN apk add --no-cache \
         git make
@@ -12,7 +12,6 @@ RUN rm Makefile && ln -s oidc-client-example.html index.html
 
 FROM halverneus/static-file-server:v1.7.0
 
-ARG VCS_REF
 ARG CODE_VERSION
 
 ENV PORT 8888
@@ -21,8 +20,10 @@ LABEL maintainer=az@zok.xyz \
     org.label-schema.name="Kopano Playground" \
     org.label-schema.description="Container for running Kopano playground applications for Kapi and OIDC" \
     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"
 
 COPY --from=builder /web /web
+
+ARG VCS_REF
+LABEL org.label-schema.vcs-ref=$VCS_REF