label weavedb image

Fixes weaveworks/scope#2610

prog/weavedb/Dockerfile

@@ -1,6 +1,7 @@
 # This is a nearly-empty image that we use to create a data-only container for persistence
 FROM scratch
 LABEL maintainer "Weaveworks Inc <help@weave.works>"
+LABEL works.weave.role=system
 ENTRYPOINT ["data-only"]
 # Work round Docker refusing to save an empty image
 COPY Dockerfile /