added paragraph on overriding the compose.yml

docs/operators/handbook.md

@@ -320,6 +320,36 @@ If you need to run a command within a running container you can use `abra app ru
 
 If you need to run a command on a container that won't start (eg. the container is stuck in a restart loop) you can temporarily disable its default entrypoint by setting it in `compose.yml` to something like ['tail', '-f', '/dev/null'], then redeploy the stack (with `--force --chaos` so you don't need to commit), then [get into the now running container](#how-do-i-attach-to-a-running-container), do your business, and when done revert the compose.yml change and redeploy again.
 
+## How can I modify/override the `compose.yml-file`?
+
+If you need a customization of the `compose`-file, e.g., override a specific, hard coded value that is not present in the sample-env, add a custom volume, or add an environment variable that the image knows but which is not (yet) included in the `compose.yml` of the recipe, you can do so by using the `COMPOSE_FILE` environment variable ([more details in Docker docs](https://docs.docker.com/compose/how-tos/environment-variables/envvars/#compose_file)).
+
+For details about how the two compose files are merged, consult the official [Docker docs](https://docs.docker.com/compose/how-tos/multiple-compose-files/merge/).
+
+If it's not a special or edge case, perhaps consider modifying the original recipe / racing a feature request so everyone can benefit from your conceptual work?
+
+### Example 
+The upstream image of your `app` allows you to modify the SMTP port with an environment variable called `SMTP_PORT`, but the recipe's maintainers didn't include it in the compose file because they didn't have in mind anyone would need a non-standard port. So you can't simply add `SMTP_PORT` to your `yourapp.example.com.env`, because it won't find its way into the running container.
+
+For a quick fix, you could now create a file, e.g. `yourapp.example.com.compose.override.yml` (naming is up to you) with the content:
+
+```
+services:
+  app:
+    environment:
+      SMTP_PORT: 25
+```
+
+and add to your `yourapp.example.com.env`
+
+```
+COMPOSE_FILE="compose.yml:../../servers/<YOUR-SERVER>/yourapp.example.com.compose.override.yml
+```
+_Make sure you include the original `compose.yml` and place the `yourapp.domain.compose.override.yml` directly alongside of your `yourapp.example.com.env`, or change the (relative) path respectively._
+
+This will now add/overwrite the `SMTP_PORT` environment variable of the `app` container.
+
+`
 ## Can I run Co-op Cloud on ARM?
 
 `@Mayel`: