Fix builds on ARM arch but keeping ARM cross builds. Fix #638.

proxy/Dockerfile.armhf

@@ -1,7 +1,5 @@
 FROM resin/rpi-raspbian:stretch
 
-RUN [ "cross-build-start" ]
-
 RUN apt-get update \
       && apt-get install -y \
         ca-certificates \
@@ -16,5 +14,3 @@ EXPOSE 8080
 COPY nginx.conf /etc/nginx/nginx.conf
 
 CMD ["nginx", "-g", "daemon off;"]
-
-RUN [ "cross-build-end" ]