Add --userns flag to support Docker daemon that enables user namespace (#533)

I got an error like this after hitting `act` command.

> Error: Error response from daemon: cannot share the host's network namespace when user namespaces are enabled

According to the document, when user namespaces are enabled on the Docker daemon,
neither host network mode and --privileged work without --userns=host. Since `act`
uses host network mode to match GitHub Actions runners, it cannot run jobs when
user namespaces are enabled. So I added the flag.

https://docs.docker.com/engine/security/userns-remap/#user-namespace-known-limitations

Co-authored-by: Casey Lee <cplee@nektos.com>

act/runner/step_context.go

@@ -234,6 +234,7 @@ func (sc *StepContext) newStepContainer(ctx context.Context, image string, cmd [
 		Stdout:      logWriter,
 		Stderr:      logWriter,
 		Privileged:  rc.Config.Privileged,
+		UsernsMode:  rc.Config.UsernsMode,
 	})
 	return stepContainer
 }