add nix steeps

.woodpecker.yml

@@ -1,5 +1,6 @@
 pipeline:
   test:
+    group: poetry
     image: python:${PYTHON_VERSION}
     pull: true
     environment:
@@ -8,6 +9,13 @@ pipeline:
       - pip install poetry
       - poetry install
       - poetry run pytest
+  nix:
+    group: nix
+    image: nixos/nix:latest
+    pull: true
+    commands:
+      - nix build
+      - nix build .#docker
 
 matrix:
   PYTHON_VERSION: