Included comment for the docker image to use

.gitlab-ci.yml

@@ -4,6 +4,9 @@
 # Dependency Scanning customization: https://docs.gitlab.com/ee/user/application_security/dependency_scanning/#customizing-the-dependency-scanning-settings
 # Note that environment variables can be set in several places
 # See https://docs.gitlab.com/ee/ci/variables/#cicd-variable-precedence
+#
+# Specify the docker image to use (only used if using docker runners)
+# See http://doc.gitlab.com/ee/ci/docker/using_docker_images.html)
 default:
   image: rikorose/gcc-cmake
 stages: