add notes to building custom image

docs/installation-containerized/v4_docker_migration.md

@@ -78,7 +78,27 @@ You will also need to update the `AA_DOCKER_TAG` to the version of V4.x you want
 
 ## (Optional) Build Custom Container
 
-If you are using a docker container with a requirements.txt, build one for v4.x as follows
+If you are using a docker container with a requirements.txt, You will need to reinstate some customizations.
+
+Modify `docker-compose.yml`, as follows.
+
+* Comment out the `image` line under `allianceauth`
+* Uncomment the `build` section
+* e.g.
+
+  ```docker
+    x-allianceauth-base: &allianceauth-base
+    # image: ${AA_DOCKER_TAG?err}
+    build:
+      context: .
+      dockerfile: custom.dockerfile
+      args:
+        AA_DOCKER_TAG: ${AA_DOCKER_TAG?err}
+    restart: always
+  ...
+  ```
+
+Now build your custom image
 
 ```shell
 docker compose pull
@@ -88,9 +108,9 @@ docker compose build
 ## Bring docker back up, migrate, collect static
 
 ```shell
-docker compose --env-file=.env up -d
+docker compose --env-file=.env up -d --remove-orphans
 
 docker compose exec allianceauth_gunicorn bash
 auth migrate
-auth collectstatic
+auth collectstatic --clear
 ```