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

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/