Some users have complained about builds using Fastlane being stuck since we shipped the Xcode 8.3 image which includes a OS bump to macOS Sierra.
The symptoms are can be many but the underlying issue is always the same: macOS Sierra changed the way code signing credentials are handled and it basically boils down to: Fastlane puts the credentials into your keychain but codesign is trying to access them. macOS Sierra considers this an access violation and denies access.
In order to work around this issue right now you can specify the keychain Fastlane match is supposed to use by declaring the FASTLANE_KEYCHAIN_NAME as well as the FASTLANE_KEYCHAIN_PASSWORD environment variables like this:
machine:
environment:
MATCH_KEYCHAIN_NAME: "circle"
MATCH_KEYCHAIN_PASSWORD: "circle"
If you run into further issues with this please let us know either here or by opening a support ticket.