Ahh, I think I know what the issue will be.
Does the build “hang” somewhere code signing related?
Xcode 8.0 was the last El Capitan based stack (https://github.com/bitrise-io/bitrise.io/blob/master/system_reports/osx-xcode-8.0.x.log#L438), 8.1 was the first Sierra based one (https://github.com/bitrise-io/bitrise.io/blob/master/system_reports/osx-xcode-8.1.x.log#L454).
There was a keychain unlock (and naming) change in Sierra, for which our own steps were upgraded to handle, but if you use your own Script or another tool for managing the signing identity / keychain unlock that might not have the Sierra change fix.
If the keychain is not unlocked properly, then Xcode will present a “Do you allow access to … ?” popup, which have to be manually accepted (clicking on “Allow” or “Always allow”).