Update u_e-docker-cust_dockerfiles.en.md

docs/manual-guides/Docker/u_e-docker-cust_dockerfiles.en.md

@@ -4,14 +4,23 @@ You need to copy the override file with corresponding build tags to the mailcow:
 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:
 
+Customize `data/Dockerfiles/$service` and build the image locally:
 ```
-docker build data/Dockerfiles/service -t mailcow/$service
+docker build data/Dockerfiles/$service -t mailcow/$service:$tag
+```
+(without a personalized :$tag docker will use :latest automatically)
+
+
+Now the created image has to be activated in docker-compose.override.yml, e.g.:
+```
+$service-mailcow:
+    build: ./data/Dockerfiles/$service
+    image: mailcow/$service:$tag
 ```
 
 Now auto-recreate modified containers:
 
 ```
 docker compose up -d
-```
+```