Make the passthrough workflow a little more intuitive

[boardfish]

@johndbritton has mentioned that having to turn on passthrough isn't terribly intuitive. We could do with an alternate solution, but I'm not sure what that might be.

[boardfish]

Nearly a year later and it's dawned on me after using this again. We could have an environment variable that marks out the names of environment variables we want to use passthrough on. If it's all, we should pass everything through.