ECR: push to repository using Dockerfile

The toolkit should have a way to build and push an image to an ECR repository by interacting with a Dockerfile. This could be a right click action 'Push to ECR...' that asks the user where it should push to. It would be nice if the toolkit also remembers which repository I selected for faster workflows.

No file or test entry point is named. Start by tracing how Dockerfile actions and ECR workflows are exposed in the VS Code toolkit, then determine how repository selection and persistence fit together. Done means a user can choose an ECR repository from a Dockerfile action, push the built image, and reuse the selected repository.

Written by the indexing model from the issue text.