doc/nextlevel-ci: added snippet to install a GitLab runner

To use the GitLab CI on a project, a server needs to install a
runner. The NetModule Know-How documentation holds already an
instruction guide about how to set up such a runner. For
completness a short summary is added to the next-level CI page.

Signed-off-by: Marc Mattmüller <marc.mattmueller@netmodule.com>

doc/src/nextlevel-ci/next-level-ci.rst

@@ -511,6 +511,62 @@ much the already heavily loaded HAC CI team.
 So the further documentation is done in :ref:`nwlCiCd`
 
 
+GitLab CI/CD - Keep in mind
+###########################
+The documentation to install a GitLab CI Running is located in the
+`know-how documentation <https://jenkins.netmodule.intranet/job/NMOS/job/generate-doc/job/develop/Know-How_20Doc/infrastructure/jenkins-buildslave.html#install-gitlab-runner>`_
+
+As a short summary see the instructions below (more details on
+`GitLab <https://docs.gitlab.com/runner/install/linux-repository.html>`_):
+
+1. Make sure you have installed the docker engine on your build server:
+
+     .. code-block:: bash
+
+         # setup the repo:
+         server:~$ curl -fsSL https://download.docker.com/linux/debian/gpg | sudo gpg --dearmor -o /usr/share/keyrings/docker-archive-keyring.gpg
+
+         # select the stable repo:
+         server:~$ echo \
+         "deb [arch=$(dpkg --print-architecture) signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/debian \
+         $(lsb_release -cs) stable" | sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
+
+         # install the engine:
+         server:~$ sudo apt update
+         server:~$ sudo apt install docker-ce docker-ce-cli containerd.io
+
+2. Install the runner on the build server:
+
+     .. code-block:: bash
+
+         # setup the repo:
+         server:~$ curl -L "https://packages.gitlab.com/install/repositories/runner/gitlab-runner/script.deb.sh" | sudo bash
+
+         # install the runner package:
+         server:~$ sudo apt-get install gitlab-runner
+
+3. Get the registration token from the GitLab CI/CD Settings page:
+
+     * Go to `Settings >> CI/CD`, expand runners and **read the token**
+
+4. Register now the runner by replacing TOKEN with the read token of the previous step:
+
+     .. code-block:: bash
+
+         # Register the runner on the server:
+         sudo gitlab-runner register \
+            -non-interactive \
+            --registration-token TOKEN \
+            --url https://gitlab.netmodule.intranet/ \
+            --tls-ca-file /etc/ssl/certs/NetModule_AG_Root_CA.crt \
+            --description "OEM-Linux1" \
+            --executor "docker" \
+            --docker-image docker:20
+
+5. Launch your pipeline of a project
+
+
+
 Sandbox Section ;-D
 ###################
 some links: