Use golang 1.12 to build in dockerfile (#6285)

Dockerfile

@@ -1,7 +1,7 @@
 
 ###################################
 #Build stage
-FROM golang:1.11-alpine3.9 AS build-env
+FROM golang:1.12-alpine3.9 AS build-env
 
 ARG GITEA_VERSION
 ARG TAGS="sqlite sqlite_unlock_notify"