Ran through a few iterations of the build script and fixed a number of bugs; I've now reached a point where all my previous code to configure Jenkins is based on the assumption that Jenkins starts up functioning normally after install, as it used to, but the behaviour has changed and there's now a randomly-created user name and password that the administrator has to use to log in from the beginning. I will need to figure out how to get around this, but it is a good feature in itself.