Add links to clarify AWS and docker credentials requirements and clarification on image: tag

What does this MR do?

• Adds an AWS documentation link to explain the permissions the ECS service needs to launch fargate containers from an AWS ECR.
• Adds an AWS documentation link to explain how to enable an ECS task to use docker registry authentication credentials for task launches.
• Adds an Important Considerations section discussing how key pass-through limitations of AWS ECS Fargate may affect the decision to implement this solution versus the GitLab Kubernetes runner on EKS.

Why was this MR needed?

• Without indications on what permissions the ECS service needs to an ECR, implementers will fail to get the results shown in the practical steps.
• It may appear to customers that the Fargate runner solution not able to handle private registries outside of ECR - including private registries on a GitLab instance.

What's the best way to test this MR?

• Configure the GitLab Fargate runner with a private registry outside of AWS.
• Configure the GitLab Fargate runner with least permissions for the ECS service.

Related issues

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