replace go path with global variable.

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

docker/golang/Dockerfile

@@ -8,4 +8,7 @@ RUN apk update \
 # install glide package management.
 RUN go get github.com/Masterminds/glide && cd $GOPATH/src/github.com/Masterminds/glide && make install
 
+RUN mkdir /app && mkdir -p $GOPATH/src/github.com/appleboy
+RUN ln -sf /app $GOPATH/src/github.com/appleboy/gorush
+
 WORKDIR $GOPATH/src/github.com/appleboy/gorush