add mattermost config

docker-compose.yml

@@ -11,13 +11,20 @@ services:
         nginx['listen_port'] = '80'
         nginx['listen_https'] = false
         gitlab_rails['gitlab_shell_ssh_port'] = ${GITLAB_SSH_PORT}
+      # If you want enable mattermost, remove this line and uncomment the following lines
+        # mattermost_external_url "http://mattermost.${GITLAB_HOSTNAME}"
+        # mattermost['gitlab_auth_endpoint'] = "http://${GITLAB_HOSTNAME}/oauth/authorize"
+        # mattermost['gitlab_token_endpoint'] = "http://${GITLAB_HOSTNAME}/oauth/token"
+        # mattermost['gitlab_user_api_endpoint'] = "http://${GITLAB_HOSTNAME}/api/v4/user"
     # Traefik labels are suggested as an example for people using Traefik,
     # remove them if you are using another reverse proxy.
     labels:
       traefik.backend: 'gitlab'
       traefik.enable: 'true'
       traefik.docker.network: 'web'
       traefik.frontend.rule: "Host:${GITLAB_HOSTNAME}"
+      # If you want enable mattermost, comment the previous line and uncomment the next line
+      # traefik.frontend.rule: "Host:${GITLAB_HOSTNAME},mattermost.${GITLAB_HOSTNAME}"
       traefik.port: '80'
     # If you don't want to use a reverse proxy (not suitable for production!)
     # ports: