Use --no-cache when installing packages

rmilter/Dockerfile

@@ -1,13 +1,11 @@
 FROM alpine:edge
 
 RUN echo "@testing http://nl.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories \
- && apk add --update \
-     rmilter@testing rsyslog \
- && rm -rf /var/cache/apk/*
- 
+ && apk add --no-cache rmilter@testing rsyslog
+
 COPY rmilter.conf /etc/rmilter.conf
 COPY rsyslog.conf /etc/rsyslog.conf
 
 COPY start.sh /start.sh
 
-CMD ["/start.sh"]
+CMD ["/start.sh"]