How to configure a sidecar container to share a folder with the main container?

Hey guys, I’m new to Rancher, and also the sidecar pattern is new to me as well. I’m trying to use a sidecar container to pull files periodically from cloud storage. And those files are configuration files of the service running in the main container. But I have no idea how can I do this. Can anybody give a hand? please be as explicit as possible since I’m really new to this. Thanks a lot!