Update .gitlab-ci.yml

.gitlab-ci.yml

+image: docker:latest
+
 variables:
   DOCKER_TAG_DEV: ${CI_COMMIT_REF_NAME}
   DOCKER_IMAGE_DEV: dpsim-dev
   GIT_SUBMODULE_STRATEGY: recursive
+  DOCKER_DRIVER: overlay2
+# When using dind, it's wise to use the overlayfs driver for
+# improved performance.
+
+services:
+- docker:dind
 
 stages:
   - prepare
@@ -11,10 +19,12 @@ stages:
 
 docker:
   stage: prepare
+  before_script:
+    - docker info
   script:
     - docker build -t ${DOCKER_IMAGE_DEV}:${DOCKER_TAG_DEV} .
   tags:
-    - shell
+    - docker
 
 build:
   stage: build