Docker sandboxes do that. They take quite bit of resources though, almost same as a standard VM.

I need to mount directories or remove after creation easily, like I do with containers.

Gateway would be good too.

Firewall needs more functionality.

You can use micro VMs like Firecraker if you're rolling out your own infra. If you're on AWS, they also have a new managed Lambda MicroVM service.

Can you share more on what functionality you'd like from firewall? Feel free to drop me a message on oryx1729@protonmail.com if you'd like an early preview of my product.