go: upgrade builder image to use go1.16 (#17897)

* go: upgrade builder image to use go1.16

build/README.md

@@ -15,5 +15,7 @@ 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 <= `5.8.0`
 - `mattermost/mattermost-build-server:feb-28-2019` which is based on Go 1.12 you can use for MM versions >= `5.9.0` <= `5.15.0`
 - `mattermost/mattermost-build-server:sep-17-2019` which is based on Go 1.12.9 you can use for MM versions >= `5.16.0`
-- `mattermost/mattermost-build-server:20200322_golang-1.14.1` which is based on Go 1.14.1 you can use for MM versions >= `5.24.0`
-- `mattermost/mattermost-build-server:20201119_golang-1.15.5` which is based on Go 1.14.6 you can use for MM versions >= `5.26.0`
+- `mattermost/mattermost-build-server:20200322_golang-1.14.1` which is based on Go 1.14.1 you can use for MM versions >= `5.24.x`
+- `mattermost/mattermost-build-server:20201023_golang-1.14.6` which is based on Go 1.14.6 you can use for MM versions >= `5.25.x`
+- `mattermost/mattermost-build-server:20201119_golang-1.15.5` which is based on Go 1.15.5 you can use for MM versions >= `5.26.x` to `5.37.x`
+- `mattermost/mattermost-build-server:20210709_golang-1.16.5` which is based on Go 1.16.5 you can use for MM versions >= `5.38.x`

build/local-test-env.sh

@@ -32,7 +32,7 @@ up()
         -e GOPATH="/go" \
         -e MM_SQLSETTINGS_DATASOURCE="postgres://mmuser:mostest@postgres:5432/mattermost_test?sslmode=disable&connect_timeout=10" \
         -e MM_SQLSETTINGS_DRIVERNAME=postgres
-        mattermost/mattermost-build-server:20201119_golang-1.15.5 bash
+        mattermost/mattermost-build-server:20210709_golang-1.16.5 bash
 }
 
 down()