add pipeline for automatic new release testing

.gitlab-ci.yml

-image: alpine
+# greater or equal to 3.13 (for git, to be able to use --merged and --no-merged in the same command)
+image: alpine:3.14
 
 before_script:
   - apk add git zip patch
+  - ./create_env.sh
 
 
 build:
@@ -10,7 +12,51 @@ build:
     - master
     - testing
   script:
-    - ./create_env.sh
+    - ./apply_patch.sh
+    - mv patched/ advtrains_luaautomation_sync/
+    - zip -r advtrains_luaautomation_sync.zip advtrains_luaautomation_sync/
+  artifacts:
+    paths:
+      - advtrains_luaautomation_sync/
+      - advtrains_luaautomation_sync.zip
+
+update:
+  stage: test
+  only:
+    - master
+    - testing
+  before_script:
+    - apk add openssh-client git
+    - eval $(ssh-agent -s)
+    - echo "${SSH_PRIVATE_KEY}" | tr -d '\r' | ssh-add - > /dev/null
+    - mkdir -p ~/.ssh
+    - chmod 700 ~/.ssh
+    - gitlab_hostname=$(echo "${CI_REPOSITORY_URL}" | sed -e 's|https\?://gitlab-ci-token:.*@||g' | sed -e 's|/.*||g')
+    - ssh-keyscan "${gitlab_hostname}" >> ~/.ssh/known_hosts
+    - chmod 644 ~/.ssh/known_hosts
+    - git config user.name "Patch Test Bot"
+    - git config user.email "patch.test.bot@example.com"
+    - TARGET_REPO=$(echo "${CI_REPOSITORY_URL}" | sed -e 's|https\?://gitlab-ci-token:.*@|ssh://git@|g')
+    - TARGET_BRANCH="ci-test-${CI_COMMIT_BRANCH}"
+  script:
+    - git checkout -B "$TARGET_BRANCH"
+    - cd advtrains
+    - git fetch
+    - TAGS="$( git tag --merged "$CI_COMMIT_BRANCH" --no-merged HEAD )"
+    - cd ..
+    - >
+      for t in $TAGS; do
+        git update-index --cacheinfo 160000,$(cd advtrains && git rev-list -n 1 tags/$t),advtrains
+        git commit -m "Test patch with $t"
+        git push -f "$TARGET_REPO" "$TARGET_BRANCH" && sleep 2
+      done
+
+build:
+  stage: test
+  only:
+    - ci-test-master
+    - ci-test-testing
+  script:
     - ./apply_patch.sh
     - mv patched/ advtrains_luaautomation_sync/
     - zip -r advtrains_luaautomation_sync.zip advtrains_luaautomation_sync/