`env()` should be moved to a hook like setup, to be run in each container context

[johnrom]

Env was useful for enabling profiler in PHP only when necessary (nimble up profile), but unfortunately XDebug v3 is no longer compatible.

I'd still like to be able to pass an argument to enable profiling with nimble up, but it would have to apply differently to each project.

Adding a hook to set up environment variables within each container's context would be a great place to move this.