Attempt better way

.gitlab-ci.yml

@@ -7,11 +7,24 @@ pages:
   stage: deploy
   script:
     - ikiwiki --setup ikiwiki.setup
-  - echo "Pages accessible through ${CI_PAGES_URL}/${CI_COMMIT_BRANCH}"
+    - echo "Pages accessible through ${CI_PAGES_URL}/${PAGES_PREFIX}"
+  variables:
+    PAGES_PREFIX: "" # no prefix by default (master)
+  pages:
+    path_prefix: "$PAGES_PREFIX"
+  environment:
+    name: "Pages ${PAGES_PREFIX}"
+    url: "${CI_PAGES_URL}/${PAGES_PREFIX}"
   artifacts:
     paths:
     - public
-  pages:
-    path_prefix: "$CI_COMMIT_BRANCH"
+  rules:
+    - if: $CI_COMMIT_BRANCH == "staging" # ensure to run on master (with default PAGES_PREFIX)
+      variables:
+        PAGES_PREFIX: '_stg' # prefix with _stg for the staging branch
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event" # conditionally change the prefix on Merge Requests
+      when: manual # run pages manually on Merge Requests
+      variables:
+        PAGES_PREFIX: 'mr$CI_MERGE_REQUEST_IID' # prefix with the mr<iid>, like `mr123`
   tags:
     - knit