fix nix store volume

docker-compose.yml

@@ -7,8 +7,11 @@ services:
     build: ./nix-builder
 
     volumes:
-      #- ./nix:/nix
+      - nix_store:/nix
       - ./workspace:/workspace
       
     environment:
-      - USER=root
+      - USER=root
+
+volumes:
+  nix_store: