Update Dockerfile to update alpine to v3.3 & fix virtual package and repository pinning on RPi

- Dockerfile now uses alpine:3.3 as base
- Dockerfile.rpi now uses v3.3/community repository without pinning
- Go package is no longer fetched using repository pinning
- Fixes problem while using repository pinning & virtual package at the same time

Dockerfile

@@ -1,10 +1,9 @@
-FROM alpine:3.2
+FROM alpine:3.3
 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
 RUN echo "@edge http://dl-4.alpinelinux.org/alpine/edge/main" | tee -a /etc/apk/repositories \
- && echo "@community http://dl-4.alpinelinux.org/alpine/edge/community" | 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

Dockerfile.rpi

@@ -3,8 +3,8 @@ 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 "@edge http://dl-4.alpinelinux.org/alpine/edge/main" | tee -a /etc/apk/repositories \
- && echo "@community http://dl-4.alpinelinux.org/alpine/edge/community" | tee -a /etc/apk/repositories \
+RUN 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

docker/build.sh

@@ -7,7 +7,7 @@ export GOPATH=/tmp/go
 export PATH=${PATH}:${GOPATH}/bin
 
 # Install build deps
-apk -U --no-progress add --virtual build-deps linux-pam-dev go@community gcc musl-dev
+apk -U --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/