dockerfile: move ARG lines to improve cache hits

Author: sameersbn

Dockerfile

@@ -1,7 +1,5 @@
 FROM ubuntu:bionic-20200219
 
-ARG BUILD_DATE
-ARG VCS_REF
 ARG VERSION=12.9.4
 
 ENV GITLAB_VERSION=${VERSION} \
@@ -63,6 +61,9 @@ COPY assets/runtime/ ${GITLAB_RUNTIME_DIR}/
 COPY entrypoint.sh /sbin/entrypoint.sh
 RUN chmod 755 /sbin/entrypoint.sh
 
+ARG BUILD_DATE
+ARG VCS_REF
+
 LABEL \
     maintainer="[email protected]" \
     org.label-schema.schema-version="1.0" \