Add Makefile for multi-platform Docker build and publish

Makefile

@@ -0,0 +1,41 @@
+# Multi-platform Docker build and publish for CyberChef
+
+IMAGE_NAME ?= ghcr.io/gchq/cyberchef
+PLATFORMS ?= linux/amd64,linux/arm64
+MONTHLY_TAG ?= $(shell date +%Y-%m)
+
+.PHONY: check-branch build publish scan
+
+check-branch:
+	@current=$$(git rev-parse --abbrev-ref HEAD); \
+	if [ "$$current" != "main" ]; then \
+		echo "Builds must run from the main branch (current: $$current)"; \
+		exit 1; \
+	fi
+
+build: check-branch
+	docker buildx build \
+		--pull \
+		--platform $(PLATFORMS) \
+		--tag $(IMAGE_NAME):latest \
+		--tag $(IMAGE_NAME):$(MONTHLY_TAG) \
+		--sbom=true \
+		--provenance=true \
+		--output=type=docker \
+		.
+
+publish: check-branch
+	docker buildx build \
+		--pull \
+		--platform $(PLATFORMS) \
+		--tag $(IMAGE_NAME):latest \
+		--tag $(IMAGE_NAME):$(MONTHLY_TAG) \
+		--sbom=true \
+		--provenance=true \
+		--push \
+		.
+	$(MAKE) scan
+
+scan:
+	docker scout cves $(IMAGE_NAME):latest --only-severity critical,high || true
+	@echo "⚠️  Review High/Critical vulnerability findings above."