Fix references to target image

Use ref_slug instead of latest
If executing on main branch, the latest tag will be equivalent to ref_slug

.gitlab-ci.yml

@@ -42,7 +42,7 @@ tag-latest:
 
 pages:
   image:
-    name: ${CI_REGISTRY_IMAGE}:latest
+    name: ${CI_REGISTRY_IMAGE}:${CI_COMMIT_REF_SLUG}
     entrypoint: [""]
   stage: deploy
   script:
@@ -59,14 +59,14 @@ test:
   script:
   - mkdir out
   - chmod 777 out 
-  - docker run --name examscan --rm -v $(pwd):$(pwd) -w $(pwd) ${CI_REGISTRY_IMAGE}:latest watermark.py tests/assets/pdfs out
+  - docker run --name examscan --rm -v $(pwd):$(pwd) -w $(pwd) ${CI_REGISTRY_IMAGE}:${CI_COMMIT_REF_SLUG} watermark.py tests/assets/pdfs out
   artifacts:
     paths:
     - out/
 
 test-coverage:
   image: 
-    name: ${CI_REGISTRY_IMAGE}:latest
+    name: ${CI_REGISTRY_IMAGE}:${CI_COMMIT_REF_SLUG}
     entrypoint: [""]
   stage: test
   script:
@@ -83,7 +83,7 @@ test-coverage:
 
 test-docs:
   image:
-    name: ${CI_REGISTRY_IMAGE}:latest
+    name: ${CI_REGISTRY_IMAGE}:${CI_COMMIT_REF_SLUG}
     entrypoint: [""]
   stage: test
   script: