Use alpine versioned and defin volumes

optional/traefik-certdumper/Dockerfile

@@ -1,8 +1,11 @@
-FROM alpine
+FROM alpine:3.8
 
 RUN apk --no-cache add inotify-tools jq openssl util-linux bash docker
 # while not strictly documented, this script seems to always(?) support previous acme.json versions too
 RUN wget https://raw.githubusercontent.com/containous/traefik/master/contrib/scripts/dumpcerts.sh -O dumpcerts.sh
 
+VOLUME ["/traefik"]
+VOLUME ["/output"]
+
 COPY run.sh /
 ENTRYPOINT ["/run.sh"]