feature: Add support for using AWS_ENDPOINT_URL_S3

[ethanholz]

I have been looking into deploying this on fly.io using their integration Tigris Object Storage integration. This integration exposes the AWS_ENDPOINT_URL_S3 variable which we could use to provide a minimal config for running this on fly.io. See here for more details: https://fly.io/docs/tigris/#create-and-manage-a-tigris-storage-bucket.

I tried my hand at adding this but I can't seem to find a way to correctly adapt the builder to use this information from the builder per the AWS docs. More than willing to help create a PR if you can help to point me in the right direction on how this would be implemented!