Deal with persistent state and/or environment dependencies

[adamcath]

ads assumes each service is stateless: nothing is persisted between down and up. But it's common to retain DB state between runs, to ease manual testing. Perhaps ads should have setup and teardown, which would operate on this state. However, a user might think "hm, this build step is slow - I'll just do it in setup" and thus muddy the distinction between caching build artifacts and persisting state. Would like a way to discourage this.

Relatedly, many services require some system dependencies to be installed before up will work. Perhaps ads should have deps which would set up your environment (or just tell you how?). This has some kind of relationship to the persistent state problem; maybe they can be unified somehow?