feat: improve Docker configuration and detection handling (#242)

- Pass `cfg` to `envcheck.CheckIfDockerRunning` function
- Add `Docker` struct to `config.go` for Docker configuration
- Update `config.example.yaml` with `docker` configuration options
- Modify `CheckIfDockerRunning` in `docker.go` to use Docker host from config if provided

Signed-off-by: appleboy <appleboy.tw@gmail.com>

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/242
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: wxiaoguang <wxiaoguang@noreply.gitea.com>
Co-authored-by: appleboy <appleboy.tw@gmail.com>
Co-committed-by: appleboy <appleboy.tw@gmail.com>

internal/pkg/config/config.example.yaml

@@ -68,3 +68,9 @@ container:
   # valid_volumes:
   #   - '**'
   valid_volumes: []
+
+docker:
+  # overrides the docker client host with the specified one.
+  # default value is the value of DOCKER_HOST environment variable.
+  # if DOCKER_HOST is not set, the default value is unix:///var/run/docker.sock
+  host: ""