add cache capability for docker

.gitlab-ci.yml

@@ -9,6 +9,10 @@ docker-build:
     DOCKER_TLS_CERTDIR: "/certs"
   before_script:
     - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
+  cache:
+    key: ${CI_PROJECT_ID}
+    paths:
+      - .docker/cache
   # All branches are tagged with $DOCKER_IMAGE_NAME (defaults to commit ref slug)
   # Default branch is also tagged with `latest`
   script: