[MM-55004] Add warning log message when app run as root (#25213)

* feat: add log warn when app is run as root user

* Modify warning message

* Move warning message condition to L415

* move check to cmd/mattermost/main.go

* Update server/channels/app/server.go

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

* Moved checkForRootUser to root.go

* format root.go

* remove unnecessary space

---------

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/cmd/mattermost/commands/root.go

@@ -4,6 +4,9 @@
 package commands
 
 import (
+	"os"
+
+	"github.com/mattermost/mattermost/server/public/shared/mlog"
 	"github.com/spf13/cobra"
 )
 
@@ -18,8 +21,18 @@ var RootCmd = &cobra.Command{
 	Use:   "mattermost",
 	Short: "Open source, self-hosted Slack-alternative",
 	Long:  `Mattermost offers workplace messaging across web, PC and phones with archiving, search and integration with your existing systems. Documentation available at https://docs.mattermost.com`,
+	PersistentPreRun: func(cmd *cobra.Command, args []string) {
+		checkForRootUser()
+	},
 }
 
 func init() {
 	RootCmd.PersistentFlags().StringP("config", "c", "", "Configuration file to use.")
 }
+
+// checkForRootUser logs a warning if the process is running as root
+func checkForRootUser() {
+	if os.Geteuid() == 0 {
+		mlog.Warn("Running Mattermost as root is not recommended. Please use a non-root user.")
+	}
+}