Added a CI job to sync images to AWS ECR Public.

[thresheek]

Proposed changes

This CI job adds a periodic (and a manual) way to sync current Docker Hub images to a Public AWS ECR registry owned by nginx.

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the CONTRIBUTING document
• I have run ./update.sh and ensured all entrypoint/Dockerfile template changes have been applied to the relevant image entrypoint scripts & Dockerfiles
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation