I understand that files / folders can be copied into a container using the command:
kubectl cp /tmp/foo_dir <some-pod>:/tmp/bar_dir
However, I am looking to do this in a yaml file
How would I go about doing this? (Assuming that I am using a deployment for the container)
The way you are going is wrong direction. Kubernetes does this with serveral ways.
first, think about configmap
https://kubernetes.io/docs/tasks/configure-pod-container/configure-pod-configmap
You can easily define the configuration files for your application running in container
If you do know the files or folders is exist on worker nodes, you can use hostPath to mount it into container with nominated nodeName: node01 in k8s yaml.
https://kubernetes.io/docs/concepts/storage/volumes/#hostpath
if the files or folders are generated temporarily, you can use emptyDir
https://kubernetes.io/docs/concepts/storage/volumes/#emptydir