feat: add configurable bind_workdir option with workspace cleanup for DinD setups (#810)

## Summary

Adds a `container.bind_workdir` config option that exposes the nektos/act `BindWorkdir` setting. When enabled, workspaces are bind-mounted from the host filesystem instead of Docker volumes, which is required for DinD setups where jobs use `docker compose` with bind mounts (e.g. `.:/app`).

Each job gets an isolated workspace at `/workspace/<task_id>/<owner>/<repo>` to prevent concurrent jobs from the same repo interfering with each other. The task directory is cleaned up after job execution.

### Configuration

```yaml
container:
  bind_workdir: true
```

When using this with DinD, also mount the workspace parent into the runner container and add it to `valid_volumes`:
```yaml
container:
  valid_volumes:
    - /workspace/**
```

*This PR was authored by Claude (AI assistant)*

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/810
Reviewed-by: ChristopherHX <38043+christopherhx@noreply.gitea.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

internal/pkg/config/config.example.yaml

@@ -103,6 +103,12 @@ container:
   require_docker: false
   # Timeout to wait for the docker daemon to be reachable, if docker is required by require_docker or act_runner
   docker_timeout: 0s
+  # Bind the workspace to the host filesystem instead of using Docker volumes.
+  # This is required for Docker-in-Docker (DinD) setups when jobs use docker compose
+  # with bind mounts (e.g., ".:/app"), as volume-based workspaces are not accessible
+  # from the DinD daemon's filesystem. When enabled, ensure the workspace parent
+  # directory is also mounted into the runner container and listed in valid_volumes.
+  bind_workdir: false
 
 host:
   # The parent directory of a job's working directory.