ci: change images

.gitlab-ci.yml

@@ -20,7 +20,7 @@ stages:
       - Dockerfile
 
 check_spelling:
-  image: alpine:latest
+  image: python:3.9
   stage: test
   before_script:
     - *install_requirements
@@ -28,7 +28,6 @@ check_spelling:
     make spell
 
 check_code:
-  image: alpine:latest
   stage: test
   script:
     liquidsoap --check src/engine.liq