feat(image): KEY-WP-0002 T01/T02/T06 — Makefile image targets, Gitea Actions workflow, README CI docs

- Makefile: add IMAGE_REGISTRY/IMAGE_REPO/IMAGE_TAG vars + image, push, image-tag targets
- .gitea/workflows/image.yaml: build+push on main push and v* tags via metadata-action
- README: Container Image section with pull/build/push/CI secret docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Makefile

@@ -1,4 +1,9 @@
-.PHONY: dev seed build test lint
+IMAGE_REGISTRY ?= 92.205.130.254:32166
+IMAGE_REPO     ?= netkingdom/key-cape
+IMAGE_TAG      ?= latest
+IMAGE          := $(IMAGE_REGISTRY)/$(IMAGE_REPO):$(IMAGE_TAG)
+
+.PHONY: dev seed build test lint image push image-tag
 
 dev:
 	docker compose -f docker-compose.dev.yml up
@@ -14,3 +19,12 @@ test:
 
 lint:
 	cd src && go vet ./...
+
+image:
+	docker build -t $(IMAGE) .
+
+push: image
+	docker push $(IMAGE)
+
+image-tag:
+	docker tag $(IMAGE) $(IMAGE_REGISTRY)/$(IMAGE_REPO):$(IMAGE_TAG)