We wanted to be able to use the IAM role provided by the EC2 instance
metadata in order to access S3 via the Minio configuration. To do this,
a new credentials chain is added that will check the following locations
for credentials when an access key is not provided. In priority order,
they are:
1. MINIO_ prefixed environment variables
2. AWS_ prefixed environment variables
3. a minio credentials file
4. an aws credentials file
5. EC2 instance metadata
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