This is a thin wrapper around the cloud-localds tool so that it can be executed in environments in which it doesn't exist and isn't easily added.
All arguments are passed through to the underlying command, so usage should be identical, with the exception of having to execute the command via Docker:
docker run --rm -v $(pwd):/data neptune-networks/cloud-localds-docker:latest
Say for example you had defined user-data, meta-data, and network-config files in the current directory, you could build the cloud-init local datastore with:
docker run --rm -v $(pwd):/data neptune-networks/cloud-localds-docker:latest -N network-config output.img user-data meta-data
Which would return output.img to be used as a NoCloud datastore.