Add Sagemaker ECR repo reference to documentation

[liwadman]

Sagemaker hosts some containers in ECR repos as per here:

We need to add it into our guidance. A template for exempting ECR repos exists, just need to call out sagemaker in writeup.

Relevant AWS docs: https://docs.aws.amazon.com/sagemaker/latest/dg-ecr-paths/ecr-us-east-1.html

[liwadman]

shipped!