update go to 1.12 (#10382)

* update go to 1.12

* update per feedback

* fix test

* revert test changes

* remove zap.AddCallerSkip(1)

build/Dockerfile.buildenv

@@ -1,3 +1,3 @@
-FROM golang:1.11
+FROM golang:1.12
 
 RUN apt-get update && apt-get install -y make git apt-transport-https ca-certificates curl software-properties-common build-essential zip wget xmlsec1

build/Jenkinsfile.pr

@@ -77,7 +77,7 @@ pipeline {
             }
 
             steps {
-                withDockerContainer(args: '-u root --privileged -v ${WORKSPACE}/src:/go/src/', image: 'mattermost/mattermost-build-server:dec-7-2018') {
+                withDockerContainer(args: '-u root --privileged -v ${WORKSPACE}/src:/go/src/', image: 'mattermost/mattermost-build-server:feb-28-2019') {
                     ansiColor('xterm') {
                         sh """
                             cd /go/src/github.com/mattermost/mattermost-server
@@ -122,7 +122,7 @@ pipeline {
                     }
                 }
 
-                withDockerContainer(args: "-u root --privileged --net ${COMPOSE_PROJECT_NAME}_mm-test -v ${WORKSPACE}/src:/go/src/", image: 'mattermost/mattermost-build-server:dec-7-2018') {
+                withDockerContainer(args: "-u root --privileged --net ${COMPOSE_PROJECT_NAME}_mm-test -v ${WORKSPACE}/src:/go/src/", image: 'mattermost/mattermost-build-server:feb-28-2019') {
                     ansiColor('xterm') {
                         sh """
                             cd /go/src/github.com/mattermost/mattermost-server

build/README.md

@@ -6,3 +6,14 @@ PRs opened against the `mattermost-server` repository will use the file called `
 
 The `Dockerfile` in this folder (`Dockerfile.buildenv`) is the build environment for our current builds you can find the docker image to downalod [here](https://hub.docker.com/r/mattermost/mattermost-build-server/tags/) or build your own.
 
+
+
+### Docker Image for building the Server
+
+We have a docker image to build `mattermost-server` and it is based on Go docker image.
+
+In our Docker Hub Repository we have the following images:
+
+- `mattermost/mattermost-build-server:dec-7-2018` which is based on Go 1.11 you can use for MM versions <= `5.9.0`
+- `mattermost/mattermost-build-server:feb-28-2019` which is based on Go 1.12
+