build: use GOPROXY and disable download on some steps (#7745)

* build: use GOPROXY

* disable download vendor on some steps

Dockerfile

@@ -3,6 +3,9 @@
 #Build stage
 FROM golang:1.12-alpine3.10 AS build-env
 
+ARG GOPROXY
+ENV GOPROXY ${GOPROXY:-direct}
+
 ARG GITEA_VERSION
 ARG TAGS="sqlite sqlite_unlock_notify"
 ENV TAGS "bindata $TAGS"