Add CI/CD configuration for SonarQube code analysis

.gitlab-ci.yml

+# vim: ts=2
+
+sonarqube-check:
+  variables:
+    SONAR_USER_HOME: "${CI_PROJECT_DIR}/.sonar"  # Defines the location of the analysis task cache
+    GIT_DEPTH: "0"  # Tells git to fetch all the branches of the project, required by the analysis task
+  cache:
+    key: "${CI_JOB_NAME}"
+    paths:
+      - .sonar/cache
+  script:
+    - sonar-scanner
+  allow_failure: true
+  only:
+    - master
+    - merge_requests