Sorry, I wasn't specific enough, I meant Java-based web apps^^
php based web apps are not 1 apt-get away either.
I don't know how many times I tried to run a php web app just to find out a random php5-* extension was missing, and then having to guess which one.
Modern applications using composer for dependencies give you this error information for free by virtue of the fact that `composer install` will fail. Most (all?) PaaS solutions take this into account and install those extensions. I know heroku does, as dokku uses the heroku buildpack :)