docs/maintainers/handbook.md aktualisiert

clarification of necessity of `command:` in case of custom entrypoint (and where to find it)

docs/maintainers/handbook.md

@@ -716,6 +716,11 @@ Please note:
 1. The `file_env` / `_FILE` hack is to pass secrets into the container runtime without exposing them in plaintext in the configuration. See [this entry](/maintainers/handbook/#exposing-secrets) for more.
 
 1. In order to pass execution back to the original entrypoint, it's a good idea to find the original entrypoint script and run it from your own entrypoint script. If there is none, you may want to reference the `CMD` definition or if that isn't working, try to actually specify `cmd: ...` in the `compose.yml` definition (there are other recipes which do this).
+  
+1. Also it might be necessary to define command: although there is an original entrypoint. That's [due to the fact](https://docs.docker.com/reference/compose-file/services/#entrypoint) that if entrypoint is non-null, Compose ignores any default command from the image, for example the `CMD` instruction in the Dockerfile.
+
+  1. Pratically you would e.g. look for the Dockerfile of the upstream image. In there you should find the docker-entrypoint.sh (or similar) and where it's located. Furthermore you find the `CMD`-line there. 
+  1. Just put in your entrypoint.sh in the last line: exec /path/to/docker-entrypoint.sh "@" (path and filename you should find in upstream Dockerfile) and insert  command:  to your service in compose.yml with the value of what you find in the CMD line of the Dockerfile.
 
 1. If you're feeling reckless, you can also use the Golang templating engine to do things conditionally.