Merge branch 'srb2ci' into 'master'

GitLabCI: use new builder image, srb2ci

See merge request STJr/SRB2!2237

.gitlab-ci.yml

@@ -3,7 +3,7 @@ variables:
   GIT_CLONE_PATH: $CI_BUILDS_DIR/$CI_CONCURRENT_ID/$CI_PROJECT_PATH
 
 default:
-  image: debian:stable-slim
+  image: git.do.srb2.org:5050/stjr/srb2ci/srb2ci:stable
 
   cache:
     - key: ccache-$CI_PROJECT_PATH_SLUG-$CI_JOB_NAME_SLUG