Document persisting /data for docker container (#160)

`/data` must be kept between container restarts.

Co-authored-by: Valentin Brandl <mail@vbrandl.net>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/160
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: techknowlogick <techknowlogick@noreply.gitea.io>
Co-authored-by: vbrandl <vbrandl@noreply.gitea.io>
Co-committed-by: vbrandl <vbrandl@noreply.gitea.io>

README.md

@@ -91,5 +91,29 @@ You can specify the configuration file path with `-c`/`--config` argument.
 ### Run a docker container
 
 ```sh
-docker run -e GITEA_INSTANCE_URL=http://192.168.8.18:3000 -e GITEA_RUNNER_REGISTRATION_TOKEN=<runner_token> -v /var/run/docker.sock:/var/run/docker.sock --name my_runner gitea/act_runner:nightly
+docker run -e GITEA_INSTANCE_URL=http://192.168.8.18:3000 -e GITEA_RUNNER_REGISTRATION_TOKEN=<runner_token> -v /var/run/docker.sock:/var/run/docker.sock -v $PWD/data:/data --name my_runner gitea/act_runner:nightly
+```
+
+The `/data` directory inside the docker container contains the runner API keys after registration.
+It must be persisted, otherwise the runner would try to register again, using the same, now defunct registration token.
+
+### Running in docker-compose
+
+```yml
+...
+  gitea:
+    image: gitea/gitea
+    ...
+
+  runner:
+    image: gitea/act_runner
+    restart: always
+    depends_on:
+      - gitea
+    volumes:
+      - ./data/act_runner:/data
+      - /var/run/docker.sock:/var/run/docker.sock
+    environment:
+      - GITEA_INSTANCE_URL=<instance url>
+      - GITEA_RUNNER_REGISTRATION_TOKEN=<registration token>
 ```