How would you do it in Python? Once you figure out how you would solve it in Python, we cal help you to build a Prefect flow out of it - feel free to share your code
Generally speaking, Prefect flows are just Python functions decorated with task and flow decorator -
this page may help you get started with flows, but if you have some more specific questions, please ask
If you are asking how to load a file from a host machine from a Docker container running the flow, you would need to mount the volume - I assume you are asking for Prefect 1.0 - in that case, you could bind mount the volume when starting the agent:
prefect agent docker start --label AGENT_LABEL --volume /local/path:/container/path