make: add docker buildx multi-arch targets (linux/amd64, linux/arm64) with setup, push, and local build

Makefile

@@ -218,11 +218,35 @@ validate-config: ## Validate environment configuration
 	fi
 
 # Docker targets
-docker-build: ## Build Docker images
-	$(call info,Building Docker images...)
+DOCKER_PLATFORMS := linux/amd64,linux/arm64
+
+docker-build: ## Build Docker images (docker-compose)
+	$(call info,Building Docker images with docker-compose...)
 	docker-compose build
 	$(call success,Docker images built)
 
+docker-buildx-setup: ## Ensure docker buildx builder exists and is bootstrapped
+	$(call info,Setting up docker buildx builder...)
+	@docker buildx create --name unitforge-builder --use 2>/dev/null || true
+	@docker buildx inspect --bootstrap
+	$(call success,Buildx builder ready)
+
+docker-buildx: docker-buildx-setup ## Build and push multi-arch image (linux/amd64, linux/arm64)
+	$(call info,Building multi-arch image with buildx...)
+	@if [ -f ".env" ]; then \
+		REGISTRY_URL=$$(grep '^CONTAINER_REGISTRY_URL=' .env | cut -d'=' -f2 | sed 's|^https\?://||'); \
+		CONTAINER_TAG=$$(grep '^CONTAINER_TAG=' .env | cut -d'=' -f2); \
+		docker buildx build --platform $(DOCKER_PLATFORMS) -t $${REGISTRY_URL}:$${CONTAINER_TAG} -t $${REGISTRY_URL}:latest --push -f Dockerfile .; \
+	else \
+		echo -e "$(YELLOW)$(WARNING_SYMBOL)$(NC) .env file not found. Set CONTAINER_REGISTRY_URL and CONTAINER_TAG to enable push."; \
+	fi
+	$(call success,Multi-arch image built and pushed)
+
+docker-buildx-local: docker-buildx-setup ## Build for current arch locally (no push)
+	$(call info,Building local image for current architecture...)
+	docker buildx build --load -t unitforge:local -f Dockerfile .
+	$(call success,Local image built: unitforge:local)
+
 # Container Registry targets
 docker-tag: ## Tag Docker images with registry URL
 	$(call info,Tagging Docker images for registry...)