Switch to ubuntu:22.04 CI image

.gitlab-ci.yml

-image: docker:stable
+image: ubuntu:22.04
 
 stages:
   - tests
@@ -6,10 +6,9 @@ stages:
 tests:
   stage: tests
   before_script:
-    - apk update
-    - apk add py3-pip
-    - pip3 install pytest
-    - pip3 install .
+    - apt update && apt install -y python3-pip
+    - pip install pytest
+    - pip install .
   script:
     - pytest tests/
   tags: