manager: use image from 'knot-resolver-ci'

This was forgotten during the CI overhaul.

manager/.gitlab-ci.yml

@@ -2,7 +2,7 @@ stages:
   - check
 
 default:
-  image: $CI_REGISTRY/knot/knot-resolver/ci/manager:knot-$KNOT_VERSION
+  image: $IMAGE_PREFIX/manager:$IMAGE_TAG
   before_script:
     - cd manager
     - poetry --version
@@ -70,3 +70,8 @@ unit:py3.12:
   <<: *unit
   variables:
     PYTHON_INTERPRETER: python3.12
+
+unit:py3.13:
+  <<: *unit
+  variables:
+    PYTHON_INTERPRETER: python3.13