Find the image you want to push
Tag your image with the ECR registry (replace with your image-id, region name and choose name for your registry)
docker tag <image-id> <aws_account_id>.dkr.ecr.<region>.amazonaws.com/<my-app>
Finally. Push the image to the registry
docker push <aws_account_id>.dkr.ecr.region.amazonaws.com/<my-app>
If you get this error
denied: Your Authorization Token has expired. Please run 'aws ecr get-login --no-include-email' to fetch a new one.
Run the command as suggested and then copy the output and run it as command (will look like docker login -u AWS -p XXXX........)