First draft of configuration proposal.

@smarterclayton 
Starting point for discussion. Feedback welcome. Also let me know where you'd like more detail.
"should commit these identifiers to version control" <-- that implies committing of "build" artifacts, which is a bit of an anti-pattern. Perhaps it'd be better to recommend using non-floating (i.e., version) tags, or accept the consequences?

Also, there aren't too many options for the TBD mechanism for tag lookup - the standard mechanism is some sort of Docker index.