Automatically trigger GitLab Pages pipelines

Whenever master or one for the v9_* branches gets updated, the current
ARM should be published on GitLab Pages.  Add a pipeline stage which
takes care of triggering GitLab Pages pipelines.  Extend the lifetime of
artifact archives containing the ARM to prevent GitLab Pages pipelines
from failing due to artifacts being unavailable.

(cherry picked from commit 31bde118db)

.gitlab-ci.yml

@@ -9,6 +9,7 @@ stages:
   - precheck
   - build
   - test
+  - push
 
 .centos-centos6-amd64: &centos_centos6_amd64_image
   image: "$CI_REGISTRY_IMAGE:centos-centos6-amd64"
@@ -190,7 +191,17 @@ docs:debian:sid:amd64:
   artifacts:
     paths:
     - doc/arm/
-    expire_in: '1 hour'
+    expire_in: '1 month'
+
+push:docs:
+  <<: *debian_sid_amd64_image
+  stage: push
+  dependencies: []
+  script:
+    - curl -X POST -F token=$GITLAB_PAGES_DOCS_TRIGGER_TOKEN -F ref=master $GITLAB_PAGES_DOCS_TRIGGER_URL
+  only:
+    - master@isc-projects/bind9
+    - /^v9_.*$/@isc-projects/bind9
 
 #build:debian:jessie:amd64:
 #  <<: *debian_jessie_amd64_image