Add DIND docker-compose example

examples/docker-compose/README.md

@@ -18,6 +18,29 @@
       - GITEA_INSTANCE_URL=<instance url>
       # When using Docker Secrets, it's also possible to use
       # GITEA_RUNNER_REGISTRATION_TOKEN_FILE to pass the location.
-      # The env var takes precedence
+      # The env var takes precedence.
+      # Needed only for the first start.
+      - GITEA_RUNNER_REGISTRATION_TOKEN=<registration token>
+```
+
+### Running `act_runner` using Docker-in-Docker (DIND) 
+
+```yml
+...
+  runner:
+    image: gitea/act_runner:latest-dind-rootless
+    restart: always
+    privileged: true
+    depends_on:
+      - gitea
+    volumes:
+      - ./data/act_runner:/data
+    environment:
+      - GITEA_INSTANCE_URL=<instance url>
+      - DOCKER_HOST=unix:///var/run/user/1000/docker.sock
+      # When using Docker Secrets, it's also possible to use
+      # GITEA_RUNNER_REGISTRATION_TOKEN_FILE to pass the location.
+      # The env var takes precedence.
+      # Needed only for the first start.
       - GITEA_RUNNER_REGISTRATION_TOKEN=<registration token>
 ```