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

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 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/