Add ability to generate automatic ARM images

plugins/rss/Dockerfile.armhf

@@ -1,6 +1,8 @@
 FROM resin/rpi-raspbian:stretch
 MAINTAINER Kristian Haugene
 
+RUN [ "cross-build-start" ]
+
 # Update packages and install software
 RUN apt-get update \
     && apt-get -y upgrade \
@@ -14,3 +16,5 @@ ENV TRANSMISSION_DOWNLOAD_DIR=/data/completed \
     RSS_URL=**None**
 
 CMD ["/etc/transmission-rss/start.sh"]
+
+RUN [ "cross-build-end" ]