Add CI pipeline

.gitlab-ci.yml

+stages:
+  - update
+  - build
+
+variables:
+  DOCKER_IMAGE: cverstege/alma9-base
+  DOCKER_TAG: latest
+
+before_script:
+  - apt-get update && apt-get install -y curl
+
+update_dockerfile:
+  stage: update
+  script:
+    - ./alma9-base/update.sh
+    - git config user.name "CI Pipeline"
+    - git config user.email "cedric.verstege@kit.edu"
+    - git add alma9-base/Dockerfile
+    - git commit -m "Update upstream to version $(grep -oP "FROM cern/alma9-base:\K[\w.-]+" "${THIS_DIR}/Dockerfile")"
+    - git push HEAD:$CI_COMMIT_REF_NAME
+    - git remote set-url --push origin "https://$CI_COMMITTER_USER_AND_TOKEN@gitlab.etp.kit.edu/cverstege/docker-images.git"
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "schedule"
+      when: always
+      allow_failure: false
+    - when: manual
+      allow_failure: true