fix: [cron] use our cache server

``docker.io`` is not responding and this might happen again anyway
and our policy is to not depend from other servers.

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>

cron/build/Dockerfile

@@ -1,4 +1,4 @@
-FROM debian:jessie
+FROM docker.0k.io/debian:jessie
 
 RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y --force-yes cron moreutils && \