Thanks for the infos @JoshPike!
Unfortunately, although auto code signing is a great feature, not every tool supports it properly yet - see for example this Ionic 2 discussion: Can I use Bitrise to automatically build (and deploy) Ionic 2 (Cordova) based apps? - #9 by viktorbenei (“ionic platform add ios in itself is not enough, as it generates an Xcode project which has incompatible code signing settings for Xcode 8.”)