Use same rpi-raspbian image for proxy

Use the same resin/rpi-raspbian:stretch image for proxy to not have to download another image and save space.

proxy/Dockerfile.armhf

@@ -1,4 +1,4 @@
-FROM resin/rpi-raspbian:jessie
+FROM resin/rpi-raspbian:stretch
 
 RUN apt-get update \
       && apt-get install -y \