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:
+    DOCKER_BUILDX_PLATFORMS: linux/amd64
+    TAG_SUFFIX: linux
+    IMAGE_BASENAME: ${IMAGE_PREFIX}/devcontainer-flutter
+    IMAGE_NAME: ${IMAGE_BASENAME}:3.24.0-${TAG_SUFFIX},${IMAGE_BASENAME}:3.24-${TAG_SUFFIX},${IMAGE_BASENAME}:3-${TAG_SUFFIX}