add multi stage build to netdisco-base

docker/netdisco-backend/Dockerfile

@@ -5,11 +5,11 @@ FROM docker.io/netdisco/netdisco-base:${TAG}-base
 LABEL org.netdisco.maintainer="The Netdisco Project"
 LABEL org.netdisco.version=${TAG}
 
-WORKDIR $NETDISCO_HOME/netdisco-mibs
+WORKDIR /home/netdisco/netdisco-mibs
 RUN curl -sL https://api.github.com/repos/netdisco/netdisco-mibs/tags | \
   jq '.[]|.tarball_url|select(test("tarball/\\d+\\.\\d+$"))' | \
   sort -rg | head -n1 | xargs -n1 curl -sL | tar --strip-components=1 -zxf -
 
-WORKDIR $NETDISCO_HOME
+WORKDIR /home/netdisco
 ENTRYPOINT ["/home/netdisco/bin/netdisco-backend"]
 CMD ["foreground"]