S3 URL format doesn't work with localstack

[shane-od]

Setting s3_default_options.endpoint to http://docker.for.mac.localhost:4566 (pointing to host machine's localstack) throws Failed to open TCP connection to my-bucket.docker.for.mac.localhost:4566 (initialize: name or service not known)

Which is obviously because that subdomain/hostname is not valid. I believe there's an alternative URL format that puts the bucket as a folder instead of a subdomain. That should be preferred for compatibility with localstack.