Fix Docker Compose command for GitHub Actions compatibility

Author: yhirose

test/Makefile

@@ -53,12 +53,12 @@ all : test test_split
 proxy : test_proxy
 	@echo "Starting proxy server..."
 	cd proxy && \
-	docker-compose up -d
+	docker compose up -d
 	@echo "Running proxy tests..."
 	./test_proxy; \
 	exit_code=$$?; \
 	echo "Stopping proxy server..."; \
-	docker-compose down; \
+	cd proxy && docker compose down; \
 	exit $$exit_code
 
 test : test.cc include_httplib.cc ../httplib.h Makefile cert.pem