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>

pkg/container/docker_run.go

@@ -44,6 +44,7 @@ type NewContainerInput struct {
 	Stderr      io.Writer
 	NetworkMode string
 	Privileged  bool
+	UsernsMode  string
 }
 
 // FileEntry is a file to copy to a container
@@ -271,6 +272,7 @@ func (cr *containerReference) create() common.Executor {
 			Mounts:      mounts,
 			NetworkMode: container.NetworkMode(input.NetworkMode),
 			Privileged:  input.Privileged,
+			UsernsMode:  container.UsernsMode(input.UsernsMode),
 		}, nil, input.Name)
 		if err != nil {
 			return errors.WithStack(err)