Update u_e-docker-cust_dockerfiles.md

docs/u_e-docker-cust_dockerfiles.md

@@ -1,3 +1,9 @@
+You need to copy the override file with corresponding build tags to the mailcow: dockerized root folder (i.e. `/opt/mailcow-dockerized`):
+
+```
+cp helper-scripts/docker-compose.override.yml.d/BUILD_FLAGS/docker-compose.override.yml docker-compose.override.yml
+```
+
 Make your changes in `data/Dockerfiles/$service` and build the image locally:
 
 ```