Document ways to backup using docker

[LongLiveCHIEF]

related #92

Regardless of if we wind up implementing support of the bundled backup restore plugin, we should document a recommended/alternative method for backups of both the container and the volume using docker commands.

[LongLiveCHIEF]

Just in case someone else gets to this before I do, here's some info that will help put together this information:

Docker has a a few good reference on this in their docs

• Docker Docs | Backup, Restore, or Migrate Data Volumes
• Docker Docs | export reference

...and this article from geek diary,How to Backup and Restore Docker Containers also has a great synopsis

[atxbyea]

Perhaps also mention about backup up persistent volumes ? I can do that if wanted, with examples of rsync \ rclone \ rsnapshot etc.