The ipfs/ipfs-cluster docker image is based on the ipfs/go-ipfs docker image. The parent image ipfs/go-ipfs exposes the port 5001 (see: https://hub.docker.com/r/ipfs/go-ipfs/~/dockerfile/) so I don’t think it is necessary that the image ipfs/ipfs-cluster expose the ports.
Maybe the issue comes from the IP which is 127.0.0.1. But as ipfs-cluster is running in the same container as ipfs it should not be the issue but it is possible that we are missing something over there.
Are you running the ipfs daemon on the same machine as the docker image? the docker image runs ipfs too, so if you are running it on the side, the docker-ipfs will not be able to start and not be available at localhost:5001.