Container: Refactor file/OCI based implementations for verifying and computing digest

[asraa]

Note: would this break if we verify in parallel different images that require changing this global variable?
I don't think we're going to need that any time soon.. unless the unit tests have to.

I wonder whether we this should be a func GetImageDigest() and have an interface that this function implement in a structure. This way anyone who needs to update the function can create their structure and set the function to whatever they want. By default, the function would be the one declared in this file.

Originally posted by @laurentsimon in #147 (comment)