Avoid nginx compilation step

[icarito]

In #3288 we added an explicit compilation step.
I believe we should be using binaries either from Debian or Phusion, not recompiling every time.
This would speed up testing and deployment.

[jywarren]

On my last publish to production just now, I see it compiled nginx twice -- once just before Step 1/19, and once on Step 19/19 : RUN passenger-config compile-nginx-engine --connect-timeout 60 --idle-timeout 60... odd!