Run as a container (#8) including Docker-in-Docker. (#84)

This adds a very simple Dockerfile and run script for running `act_runner` as a container.

It also allows setting `Privileged` and `ContainerOptions` flags via the new config file when spawning task containers.  The combination makes it possible to use Docker-in-Docker (which requires `privileged` mode) as well as pass any other options child Docker containers may require.

For example, if Gitea is running in Docker on the same machine, for the `checkout` action to behave as expected from a task container launched by `act_runner`, it might be necessary to map the hostname via something like:

```
container:
  network_mode: bridge
  privileged: true
  options: --add-host=my.gitea.hostname:host-gateway
```

> NOTE: Description updated to reflect latest code.
> NOTE: Description updated to reflect latest code (again).

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/84
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: Thomas E Lackey <telackey@bozemanpass.com>
Co-committed-by: Thomas E Lackey <telackey@bozemanpass.com>

internal/pkg/config/config.go

@@ -35,7 +35,9 @@ type Config struct {
 	} `yaml:"cache"`
 	Container struct {
 		NetworkMode string `yaml:"network_mode"`
-	}
+		Privileged  bool   `yaml:"privileged"`
+		Options     string `yaml:"options"`
+	} `yaml:"container"`
 }
 
 // LoadDefault returns the default configuration.