Update after actually trying to symlink docker files

srvr-doc.md

@@ -55,7 +55,7 @@ two servers are relevant
 3. In /etc/systemd/system.conf: set CPU affinity
 4. restore /etc/gitlab-runner/config.toml
 5. restore /etc/sysctl/perf.conf
-6. symlink /var/lib/docker/ to /local/docker. This works with the following script:
+6. symlink /var/lib/docker/ to /local/docker. This maybe works with the following script. But last time the move actually destroyed the docker installation (which could be fixed by reinstalling docker-ce).
 ```
 systemctl stop docker
 mv /var/lib/docker /local/docker
@@ -76,3 +76,23 @@ docker login
 docker push ralfjung/opam-ci:opam2
 ```
 
+## Testing the gitlab runner
+The gitlab runner can be tested by creating a `.gitlab-ci.yml` file in a new directory,
+```
+image: ralfjung/opam-ci:opam2
+
+stages:
+  - test
+
+my_project_test:
+  stage: test
+  script:
+    - echo 1
+```
+and then running
+```
+git init
+git add .gitlab-ci.yml
+git commit -am "init"
+gitlab-runner exec docker my_project_test
+```