Updated RFS Docker Compose to handle AWS Creds; fix S3Repo bug

[chelma]

Description

• Updated the DockerCompose file for RFS to accept AWS Creds for the containers
• Updated RFS README to explain how to leverage the new behavior
• Fixed a bug in the S3Repo class' handling of S3 URI's (it was failing to handle passing in the bucket root path as an acceptable repo location)
• Unit tested the S3Repo class

Issues Resolved

Done while working on https://opensearch.atlassian.net/browse/MIGRATIONS-1600

Testing

• Ran the new unit tests (./gradlew tests)
• Ran the RFS process from snapshot through reindexing using the docker compose setup

Check List

• New functionality includes testing
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented
• Commits are signed per the DCO using --signoff

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[mikaylathompson]

is there a reason you're not using the typical env var naming conventions locally? (e.g. not - AWS_ACCESS_KEY_ID=${AWS_ACCESS_KEY_ID} and the corresponding change in the documentation). Not a big deal, but I'm not sure I understand the reason.

[chelma]

It's so that you can use a different set of credentials on the host machine if you want to. I was having trouble with my host machine (mac laptop) picking up and using the creds intended for the RFS Containers, which was pretty annoying because that meant that when I ran the AWS CLI to get renewed creds for the Containers using STS, the CLI would use the expired credential set I was trying to renew! 🙃