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

webapp/channels/build/Dockerfile

@@ -0,0 +1,3 @@
+FROM node:14.16
+RUN apt-get update && apt-get install -y make gcc nasm libpng-dev
+CMD [ "node" ]