allow multiple cache images for one service

[dreyks]

First image that gets successfully pulled stops the chain for this service. The idea is to have

cache-from:
  - app:myrepo/myapp:${BUILDKITE_BRANCH}
  - app:myrepo/myapp:latest

so that subsequent builds for a branch will use the cache for this specific branch

[dreyks]

@lox any thoughts?

[keithpitt]

This change looks good to me! I'll poke @lox about it today and try it get it merged.

[lox]

I was hesitant to merge because I've made some pretty major changes to the cache-from mechanism in several if my WIP PR's, but I reckon the utility of the change outweighs those concerns. Sorry for the delay @dreyks!

[lox]

I'll follow up with some docs changes to reflect.

[toolmantim]

This was released in v2.6.0 (🎄 Holiday Cheer) 🎉