Hey,
this looks to me completely unrelated to ownCloud. I think you should ask questions about Docker and how to work with docker in https://forums.docker.com/ where you can find way more experienced users with Docker then in a forums of the software running within your Docker environment. 
As Docker is a widely used software and you’re probably not the first one who needs to migrate the docker storage engine on a live environment a search on e.g. google might even lead you to what you’re looking for.