Add ability to generate automatic ARM images

proxy/Dockerfile.armhf

@@ -1,5 +1,7 @@
 FROM resin/rpi-raspbian:stretch
 
+RUN [ "cross-build-start" ]
+
 RUN apt-get update \
       && apt-get install -y \
         ca-certificates \
@@ -15,3 +17,4 @@ COPY nginx.conf /etc/nginx/nginx.conf
 
 CMD ["nginx", "-g", "daemon off;"]
 
+RUN [ "cross-build-end" ]