ci: pull base images from the ECR public mirror (#4)

Docker Hub org access tokens (OATs) are scoped to the org's repos, so once
buildx is authenticated with the OAT it can't pull public base images
(docker.io/library/python -> 401 'access token has insufficient scopes'),
breaking the release build. Pull the python base from
public.ecr.aws/docker/library (anonymous, byte-identical mirror, no Docker Hub
auth) — matching RIDI's monorepo convention. Push to ridi/ministack still uses
the OAT.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>