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 <[email protected]>

cmd/input.go

@@ -25,6 +25,7 @@ type Input struct {
 	insecureSecrets bool
 	defaultBranch   string
 	privileged      bool
+	usernsMode      string
 }
 
 func (i *Input) resolve(path string) string {