Use passed context in NewStepsDocker.Exec()

I found this while working on Prevent API client from hanging when server is ... (step-runner!302 - merged)

As far as I've been able to determine, the only difference between the context embedded in the executor and the one passed to startAndWatchContainer is that the latter wraps the former in a cancellable context. I don't see a good reason to use the former over the latter, and using a single one keeps things clearer. Like, I wrote this code and am now confused why we use the executors context in that one place.

All unit/integration tests continue to pass.