add .gitlab-ci.yml

.gitlab-ci.yml

+include:
+- project: docker-images/templates
+  ref: master
+  file: devcontainer_images.gitlab-ci.yml
+
+build:
+  extends: .build_image_job_template
+  variables:
+    IMAGE_BASENAME: ${IMAGE_PREFIX}/devcontainer-dotnet
+    IMAGE_NAME: ${IMAGE_BASENAME}:8.0