Add go environment vendor directory support (#781)

* Add go environment vendor directory support
* Use symbolink instead of copying files to gopath
* Add go vendor example package & test

environments/go/builder/Dockerfile

@@ -5,9 +5,9 @@ FROM ${BUILDER_IMAGE}
 
 FROM golang:${GO_VERSION}
 
-COPY --from=0 /builder /builder
-
 ENV GOPATH /usr
+WORKDIR ${GOPATH}
 
+COPY --from=0 /builder /builder
 ADD build.sh /usr/local/bin/build