I completed the integration of the latest Google Tag Manager (v5) for iOS together with Firebase (https://developers.google.com/tag-manager/ios/v5/).
The big change here is that the default container file is not binary anymore, it is plain JSON.
The integration requires that you have a folder (not group!) with the name "container" inside your app workspace. Within this folder the container file should be located. This raises my issue: We have two different GTM Containers, one for the testing/development app and one for production.
You should be able to configure an XCode "run script" build step that clears the container directory and copies the correct container into place.