reference docker images at docker.io

docker/netdisco-backend/Dockerfile

@@ -1,5 +1,8 @@
 # vim: ft=Dockerfile
-FROM netdisco-base:latest
+ARG TAG=master
+FROM docker.io/netdisco:${TAG}-base
+
+VOLUME ["/home/netdisco/environments", "/home/netdisco/nd-site-local"]
 
 USER netdisco:netdisco
 WORKDIR $NETDISCO_HOME/netdisco-mibs
@@ -8,6 +11,5 @@ RUN curl -s https://api.github.com/repos/netdisco/netdisco-mibs/tags | \
   sort -rg | head -n1 | xargs -n1 curl -L | tar --strip-components=1 -zxf -
 
 WORKDIR $NETDISCO_HOME
-VOLUME ["/home/netdisco/environments", "/home/netdisco/nd-site-local"]
 ENTRYPOINT ["/home/netdisco/bin/netdisco-backend"]
 CMD ["foreground"]