Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/build/README.md

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+## About this folder
+
+This folder contains some files that we use to build the `mattermost-server` and other files like privacy policy and licenses.
+
+The `Dockerfile` in this folder (`Dockerfile.buildenv`) is the build environment for our current builds you can find the docker image to download [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 <= `5.8.0`
+- `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.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:20210810_golang-1.16.7` which is based on Go 1.16.X you can use for MM versions >= `5.38.x`