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I don’t get it. Where exactly are you trying to use that docker cp? I hope not in the “command” parameter in the compose file. That is for commands running inside the container.
And you don’t even need to copy if you can mount as you mount volumes. There are some examples here:
https://docs.docker.com/compose/compose-file/05-services/#volumes
Never copy files into a running container, especially not dependencies. Everything will disappear when you delete the container. You could create your own Dockerfile and build your own image that already has that dependency.