Build/push images

- Default builds linux/amd64 + linux/arm64 and pushes.
- Default REGISTRY is `gitea-http.taildb3494.ts.net/will/agentmon` (your Gitea registry).

Examples:

- Push to default image names:
  TAG=dev ./build/build-images.sh

- Push to your registry:
  REGISTRY=registry.docker-registry.svc.cluster.local:5000 TAG=dev ./build/build-images.sh

- Build locally without pushing (single-arch only; Docker can’t `--load` multi-arch):
  PLATFORM=linux/amd64 PUSH_ARGS=--load TAG=dev ./build/build-images.sh