CI: make sure we can build Docker image

.gitlab-ci.yml

@@ -22,3 +22,41 @@ test:
     only:
         - merge_requests
         - tags
+
+# Make sure we can build a docker image
+# It's cached for later jobs
+build_docker:
+    stage: test
+    image: docker:19.03.12
+    services:
+        - docker:19.03.12-dind
+    script:
+        # Try to load latest branch image from local tar or from registry
+        - docker load ci-exports/$CI_COMMIT_REF_SLUG.tar || docker pull $CI_REGISTRY_IMAGE:latest || true
+        - docker build --cache-from $CI_REGISTRY_IMAGE:latest --tag $CI_REGISTRY_IMAGE:latest .
+        - mkdir -p ci-exports/
+        - docker save $CI_REGISTRY_IMAGE:latest > ci-exports/$CI_COMMIT_REF_SLUG.tar
+    variables:
+        # When using dind service, we need to instruct docker to talk with
+        # the daemon started inside of the service. The daemon is available
+        # with a network connection instead of the default
+        # /var/run/docker.sock socket. Docker 19.03 does this automatically
+        # by setting the DOCKER_HOST in
+        # https://github.com/docker-library/docker/blob/d45051476babc297257df490d22cbd806f1b11e4/19.03/docker-entrypoint.sh#L23-L29
+        #
+        # The 'docker' hostname is the alias of the service container as described at
+        # https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services.
+        #
+        # Specify to Docker where to create the certificates, Docker will
+        # create them automatically on boot, and will create
+        # `/certs/client` that will be shared between the service and job
+        # container, thanks to volume mount from config.toml
+        DOCKER_TLS_CERTDIR: "/certs"
+        # Use TLS https://docs.gitlab.com/ee/ci/docker/using_docker_build.html#tls-enabled
+        DOCKER_HOST: tcp://docker:2376
+    cache:
+        # The same key should be used across branches
+        key: "$CI_COMMIT_REF_SLUG"
+        paths:
+            - ci-exports/*.tar
+