[easy] ci: Add secret detection

Author: JanEbbing

.gitlab-ci.yml

@@ -3,6 +3,9 @@ include:
   - project: '${CI_PROJECT_NAMESPACE}/ci-libs-for-client-libraries'
     file:
       - '/${CI_PROJECT_NAME}/.gitlab-ci.yml'
+  - project: 'deepl/ops/ci-cd-infrastructure/gitlab-ci-lib'
+    file:
+      - '/templates/.secret-detection.yml'
 
 # Global --------------------------
 
@@ -63,6 +66,16 @@ license_check_manual:
   rules:
     - if: $CI_PIPELINE_SOURCE != "schedule"
 
+secret_detection:
+  extends: .secret-detection
+  stage: check
+  image: !reference [.secret-detection, image]
+  variables:
+    SECRET_DETECTION_HISTORIC_SCAN: "true"
+  before_script:
+    - echo "overriding default before_script..."
+  rules:
+    - if: $CI_MERGE_REQUEST_ID
 
 # stage: build ----------------------