feat: Migrate docker preview repo stuff in monorepo (#25698)

* feat: Migrate docker preview repo stuff in monorepo

* docs: Update docs [skip ci]

* config: Update configuration to minimum [skip ci]

server/build/docker-preview/docker-entry.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+# Copyright (c) 2016 Mattermost, Inc. All Rights Reserved.
+# See License.txt for license information.
+
+echo "Starting PostgreSQL"
+docker-entrypoint.sh -c 'shared_buffers=256MB' -c 'max_connections=300' &
+
+until pg_isready -hlocalhost -p 5432 -U "$POSTGRES_USER" &> /dev/null; do
+	echo "postgres still not ready, sleeping"
+	sleep 5
+done
+
+echo "Updating CA certificates"
+update-ca-certificates --fresh >/dev/null
+
+echo "Starting platform"
+cd mattermost
+exec ./bin/mattermost --config=config/config_docker.json