AWS ECR docs

[nick-youngblut]

As a new user of vscode-docker, I've found it hard to figure out how to use vscode-docker with AWS ECR, which seems to be possible (e.g., #3202), but there doesn't seem to be much in regards to docs (besides docker-push-ecr-image, but that is CLI-based and not using the vscode-docker GUI features).

More docs on using vscode-docker with AWS ECR would be appreciated.

[bwateratmsft]

AWS has a custom auth mechanism that isn't supported by the Docker extension at this time, so I don't think it's currently possible to connect to ECR directly with the extension. I'll resolve this as a dupe of #869 and link this item in there.