*: track upstream PRs

[cyphar]

There are several components of umoci that are not very well defined by the image-spec specification. I'm working on improving this upstream, so that we can be sure that umoci is actually doing thing correctly (because we defined what is the correct way of doing them). Here's a currently list of upstream PRs:

• conversion: add document about image -> runtime configuration image-spec#492 -- conversion.md
• config: forbid snapshotting of data volumes image-spec#504 -- Forbid Config.Volumes snapshotting.

This is quite important for us.