Connect ECS Fargate to S3

I need my AWS ECS service, which runs on Fargate, to both read and write to a single S3 bucket. I want to accomplish this strictly through IAM Roles—no access keys or bucket-level policies. An IAM Role that should be used already exists; it simply needs the correct permissions and to be attached to the ECS tasks or service.

Scope of work
• Review the current IAM Role and add the least-privilege policy that grants full read/write access to the target bucket (and nothing more).
• Update the ECS task definition or service so that Fargate tasks assume this role automatically.
• Validate the setup by running a quick test from within a task/container, confirming that objects can be uploaded and retrieved.