Add postinstall step

[tracker1]

AS the initial install won't actually run the postinstall, running the postinstall would probably be a good idea after copying the rest of the application. This would be much closer to the expected behavior.

[chorrell]

We're more than likely going to deprecate the onbuild image at some point in the future.

See: docker-library/official-images#2076

[tracker1]

Kind of sad to see the move to push the -onbuild images out... They've been great, for me in terms of getting a project up/deployed quickly in, for example dokku. I mean, yeah, I can just put all the instructions that would be in -onbuild into the Dockerfile, beyond that, my build is usually a little more complex... that said, the onbuild images have been greatly appreciated.

[chorrell]

Closing this out since onbuild variants are deprecated.