add Gitlab-CI config

.gitlab-ci.yml

+---
+# A pipeline is composed of independent jobs that run scripts, grouped into stages.
+# Stages run in sequential order, but jobs within stages run in parallel.
+#
+# For more information, see: https://docs.gitlab.com/ee/ci/yaml/index.html#stages
+
+image: debian:stable-slim
+
+stages:          # List of stages for jobs, and their order of execution
+  - test
+
+default:
+  tags:
+    - docker
+
+before_script:
+  - apt-get update
+  - apt-get -y install shellcheck xargs
+
+test-syntax-job:   # This job runs in the test stage.
+  stage: test    # It only starts when the job in the build stage completes successfully.
+  script:
+    - find ./ -name *.sh | xargs shellcheck --color=always --shell=bash --external-sources