[MISC] Add build instructions in Compose

pica-mattermost/docker-compose.yml

@@ -17,6 +17,7 @@ volumes:
 services:
   mattermost:
     image: registry.picasoft.net/pica-mattermost:5.23.0
+    build: .
     container_name: mattermost-app
     links:
       - mattermost-db:mattermost-db