Dockerfile & Dockerfile.pi updates

- Upgrade of gosu to v1.7
- Change in docker/build.sh to use `--no-cache` to prevent APKINDEX creation when installing dev dependencies
- Manual upgrade of Alpine on Raspberry Pi when building to make sure the environment is the same as the standard Dockerfile

Dockerfile

 MAINTAINER jp@roemer.im
 
 # Install system utils & Gogs runtime dependencies
-ADD https://github.com/tianon/gosu/releases/download/1.6/gosu-amd64 /usr/sbin/gosu
+ADD https://github.com/tianon/gosu/releases/download/1.7/gosu-amd64 /usr/sbin/gosu
 RUN chmod +x /usr/sbin/gosu \
  && apk --no-cache --no-progress add ca-certificates bash git linux-pam s6 curl openssh socat
 

Dockerfile.rpi

 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/main/"      | tee /etc/apk/repositories    \
+ADD https://github.com/tianon/gosu/releases/download/1.7/gosu-armhf /usr/sbin/gosu
+RUN chmod +x /usr/sbin/gosu \
+ && 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 \
- && chmod +x /usr/sbin/gosu
+ && apk -U --no-progress upgrade && rm -f /var/cache/apk/APKINDEX.* \
+ && apk --no-cache --no-progress add ca-certificates bash git linux-pam s6 curl openssh socat
 
 ENV GOGS_CUSTOM /data/gogs
 

docker/build.sh

 export PATH=${PATH}:${GOPATH}/bin
 
 # Install build deps
-apk -U --no-progress add --virtual build-deps linux-pam-dev go gcc musl-dev
+apk --no-cache --no-progress add --virtual build-deps linux-pam-dev go gcc musl-dev
 
 # Init go environment to build Gogs
 mkdir -p ${GOPATH}/src/github.com/gogits/