Docs about how to generate config for act runner with docker and setup it with docker-compose (#25256)

In this pull request, the following changes are addressed:

- State user should create `config.yaml` before start container to avoid
errors.
- Provided instructions to deploy runners using docker compose.

docs/content/doc/usage/actions/act-runner.en-us.md

@@ -76,6 +76,12 @@ The default configuration is safe to use without any modification, so you can ju
 ./act_runner --config config.yaml [command]
 ```
 
+You could also generate config file with docker:
+
+```bash
+docker run --entrypoint="" --rm -it gitea/act_runner:latest act_runner generate-config > config.yaml
+```
+
 When you are using the docker image, you can specify the configuration file by using the `CONFIG_FILE` environment variable. Make sure that the file is mounted into the container as a volume:
 
 ```bash
@@ -172,6 +178,27 @@ It is because the act runner will run jobs in docker containers, so it needs to
 As mentioned, you can remove it if you want to run jobs in the host directly.
 To be clear, the "host" actually means the container which is running the act runner now, instead of the host machine.
 
+### Set up the runner using docker compose
+
+You could also set up the runner using the following `docker-compose.yml`:
+
+```yml
+version: "3.8"
+services:
+  runner:
+    image: gitea/act_runner:nightly
+    environment:
+      CONFIG_FILE: /config.yaml
+      GITEA_INSTANCE_URL: "${INSTANCE_URL}"
+      GITEA_RUNNER_REGISTRATION_TOKEN: "${REGISTRATION_TOKEN}"
+      GITEA_RUNNER_NAME: "${RUNNER_NAME}"
+      GITEA_RUNNER_LABELS: "${RUNNER_LABELS}"
+    volumes:
+      - ./config.yaml:/config.yaml
+      - ./data:/data
+      - /var/run/docker.sock:/var/run/docker.sock
+```
+
 ### Configuring cache when starting a Runner using docker image
 
 If you do not intend to use `actions/cache` in workflow, you can ignore this section.