All these messages (and the error «The file “/composer.json” doesn’t exist» message) are generated by an unknown black-box closed source script/program:
Building application ‘mymagento’ (runtime type: php:7.0, tree ID: 4be8741)
Generating runtime configuration.
Moving the application to the output directory
W: Overriding path robots.txt that already exists in destination.
Prewarming composer cache.
Pre-downloaded 2 packages referenced in composer.lock
These strings are absent in your beta and production websites source code.
So the only way to investigate the case and this unknown black-box closed source program requirements is to contact the program author - Magento Enterprise team.
Ask them:
- Where is exactly the «The file “/composer.json” doesn’t exist» message generated?
- Why is the message generated? What are the hidden requirements about such «composer.json» files?