Update Dockerfile.rpi to better match Dockerfile:

- Dockerfile.rpi now uses hypriot/rpi-alpine-scratch as base (build script are available w/ a better maintainer & more updates)
- Dockerfile.rpi updates alpine from v3.2 to v3.3 to be on par with Dockerfile

Dockerfile.rpi

@@ -1,9 +1,10 @@
-FROM sander85/rpi-alpine:latest
+FROM hypriot/rpi-alpine-scratch:v3.2
 MAINTAINER jp@roemer.im, raxetul@gmail.com
 
 # Install system utils & Gogs runtime dependencies
 ADD https://github.com/tianon/gosu/releases/download/1.6/gosu-armhf /usr/sbin/gosu
-RUN echo "http://dl-4.alpinelinux.org/alpine/v3.3/community/" | tee -a /etc/apk/repositories \
+RUN echo "http://dl-4.alpinelinux.org/alpine/v3.3/main/"      | tee /etc/apk/repositories    \
+ && echo "http://dl-4.alpinelinux.org/alpine/v3.3/community/" | tee -a /etc/apk/repositories \
  && echo "@edge http://dl-4.alpinelinux.org/alpine/edge/main" | tee -a /etc/apk/repositories \
  && apk -U --no-progress upgrade \
  && apk -U --no-progress add ca-certificates bash git linux-pam s6@edge curl openssh socat \