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>

cmd/root.go

@@ -47,6 +47,7 @@ func Execute(ctx context.Context, version string) {
 	rootCmd.Flags().StringVarP(&input.eventPath, "eventpath", "e", "", "path to event JSON file")
 	rootCmd.Flags().StringVar(&input.defaultBranch, "defaultbranch", "", "the name of the main branch")
 	rootCmd.Flags().BoolVar(&input.privileged, "privileged", false, "use privileged mode")
+	rootCmd.Flags().StringVar(&input.usernsMode, "userns", "", "user namespace to use")
 	rootCmd.PersistentFlags().StringVarP(&input.actor, "actor", "a", "nektos/act", "user that triggered the event")
 	rootCmd.PersistentFlags().StringVarP(&input.workflowsPath, "workflows", "W", "./.github/workflows/", "path to workflow file(s)")
 	rootCmd.PersistentFlags().StringVarP(&input.workdir, "directory", "C", ".", "working directory")
@@ -260,6 +261,7 @@ func newRunCommand(ctx context.Context, input *Input) func(*cobra.Command, []str
 			InsecureSecrets: input.insecureSecrets,
 			Platforms:       input.newPlatforms(),
 			Privileged:      input.privileged,
+			UsernsMode:      input.usernsMode,
 		}
 		r, err := runner.New(config)
 		if err != nil {