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/runner.go

@@ -31,6 +31,7 @@ type Config struct {
 	InsecureSecrets bool              // switch hiding output when printing to terminal
 	Platforms       map[string]string // list of platforms
 	Privileged      bool              // use privileged mode
+	UsernsMode      string            // user namespace to use
 }
 
 type runnerImpl struct {