* feat(ci): add docker build and ECR image publish reusables
Image repos need a registry-free build check and an ECR publish that stops
at a mutable tag, without an ECS or Lambda update.
* fix(ci): retry ECR digest checks when describe-images fails
A tag that is not visible yet makes the AWS CLI exit non-zero, and set -e
was aborting the retry loop on that first error.
