update base image used

[georschi]

update base image used in DockerFile. public aws should be used as when the reader tries building the image on the cloud on their own account if not authenticated on dockerhub, they may be throttled based on the low daily limit of dockerhub

[athewsey]

I think ECR public still needs users to get a docker CLI auth token right? If so, it might be useful to add this step to the README accompanying each affected sample - to make sure users who aren't familiar with the process are able to build the images as documented?