Add GitLab pipeline, gitlab stack docs and root include

gitlab/README.md

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+# GitLab stack (self-hosted)
+
+GitLab CE and its container registry behind Traefik (TLS), plus a Docker-executor runner on the same network (`web_net`).
+The pipeline for this repo is in `.gitlab-ci.yml` in this folder (included from the repo root).
+
+Register a runner (create it in Admin > CI/CD > Runners first, then use its token):
+
+    docker exec gitlab-runner gitlab-runner register --non-interactive \
+      --url http://gitlab:80 --token <RUNNER_TOKEN> \
+      --executor docker --docker-image docker:cli \
+      --docker-network-mode web_net \
+      --docker-volumes /var/run/docker.sock:/var/run/docker.sock \
+      --clone-url http://gitlab:80
+
+Where registry.gitlab.com is unreachable, add this to the runner's [runners.docker] section in config.toml:
+`helper_image = "gitlab/gitlab-runner-helper:x86_64-latest"`