add labels and fix tag refs

docker/netdisco-base/Dockerfile

@@ -1,6 +1,10 @@
 # vim: ft=Dockerfile
+ARG TAG=master
 FROM debian:stable-slim
 
+LABEL org.netdisco.maintainer="The Netdisco Project"
+LABEL org.netdisco.version=${TAG}
+
 RUN bash -c 'mkdir -p /usr/share/man/man{1..8}' && \
     apt-get -yq update && \
     apt-get install -yq --no-install-recommends \