postinst.sh lets the early parts define a few hooks to control behavior:
* After it installs the core configuration files (database.yml, application.yml, and production.rb) to /etc/arvados/server, it calls setup_extra_conffiles. By default this is a noop function (in step2.sh).
-* Before it restarts nginx, it calls setup_before_nginx_restart. By default this is a noop function (in step2.sh). API server defines this to set up the internal git repository, if necessary.
* $RAILSPKG_DATABASE_LOAD_TASK defines the Rake task to load the database. API server uses db:structure:load. Workbench doesn't set this, which causes the postinst to skip all database work.