Hello,
The command key of a docker-compose file overrides the command of the container, it is not “prepended”: https://docs.docker.com/compose/compose-file/#command
You need to update the Dockerfile in the box-ftp folder and change the CMD or ENTRYPOINT here to point to a shell script that will sleep 5 then run the command it is supposed to run.
If sleep is not available, you need to install it in the Dockerfile. On debian it is in the coreutils package for example.