[.gitlab-ci.yml] [test] Do not run 'build' if Dockerfile has not changed

Also, update some comments.
Line 50: `- "pica-ci-base/**"` changed to `- "pica-ci-base/Dockerfile"` 
in order to match comment

.gitlab-ci.yml

@@ -47,7 +47,7 @@ pica-ci-base:
   rules:
     - if: '$CI_COMMIT_BRANCH == "master" || $CI_COMMIT_BRANCH == "dev-ci"'
       changes:
-        - "pica-ci-base/**"
+        - "pica-ci-base/Dockerfile"
       when: manual
 
 # Create a file a few environment variables added (which Dockerfile has been modified, the complete name of the image...)
@@ -59,8 +59,8 @@ set-variables:
   script:
     - chmod +x ./get-modified-image.sh
     - ./get-modified-image.sh
-  # If the Dockerfile is changed, force the build.
-  # Otherwise, may be triggered manually.
+  # If there are changes in any of the files and folders of 'pica-*' or
+  # 'meta-*', then propose a manual build
   rules:
     - changes:
       - "pica-*/**"
@@ -73,7 +73,7 @@ build:
   tags: [build]
   image: $REGISTRY_PROD/pica-ci-base
   before_script:
-    # First login on the production registry, in case the image is based on a other registry image
+    # First login on the production registry, in case the image is based on another registry image
     - echo $REGISTRY_PROD_PASSWORD | docker login $REGISTRY_PROD -u $REGISTRY_PROD_USERNAME --password-stdin
     - source variables
   script:
@@ -85,17 +85,18 @@ build:
     - docker push $MODIFIED_IMAGE_FULL_TEST
   after_script:
     - docker logout $REGISTRY
-  # If the Dockerfile is changed, force the build.
-  # Otherwise, may be triggered manually.
+  # Build if Dockerfile changed and previous stage completed successfully
+  # Manual build for other changes commented out in order to not block
+  # the pipeline (see issue 34077, link on header)
   rules:
     - changes:
       - "pica-*/Dockerfile"
       - "meta-*/Dockerfile"
       when: on_success
-    - changes:
-      - "pica-*/**"
-      - "meta-*/**"
-      when: manual
+    # - changes:
+    #   - "pica-*/**"
+    #   - "meta-*/**"
+    #   when: manual
     - when: never
 
 # Run CoreOS' Clair and make the CI failed if a critical vulnerability isn't in the whitelist