Add to docs information about forbidden envs

[diogomelodantas]

Add to documentation information about variables that can be used by the workload like AWS_ACCESS_KEY and SECRET_ACCESS_KEY.

[fagiani]

I believe we can do better here: WARN boldly in logs if we find them before pack is triggered and allow a override build variable like MAESTRO_IGNORE_IAM_KEYS to discard them right before as it would be a common case to use those at runtime.