# What is a container?

A container is a process the Linux kernel isolates with its own filesystem, network and process view, sharing the host's kernel.

**Runtime runs each sandbox as a Firecracker microVM instead, with its own
kernel, and Docker runs inside it after `sudo enable-docker`.** You keep the
container tools you know, and the code in them sits behind a virtual machine
boundary rather than the host's kernel ([the sandbox environment](/docs/sandbox-environment)).

## What makes a container

Docker's own introduction describes containers as "isolated processes for each
of your app's components": self-contained, isolated, independent and portable.
The isolation comes from features of the Linux kernel, not from separate
hardware:

- **Namespaces** give the process its own view of process IDs, mounts,
  network, users and hostname ([Linux namespaces](/glossary/linux-namespaces)).
- **Control groups** cap and count its CPU, memory and I/O
  ([cgroups](/glossary/cgroups)).
- **seccomp** filters which system calls it may make
  ([seccomp](/glossary/seccomp)).
- An **image** supplies its filesystem: an application and everything it needs,
  in layers.

## Container or virtual machine

Docker's page puts the difference in one line each: "a VM is an entire
operating system with its own kernel", while "a container is simply an isolated
process with all of the files it needs to run". Sharing the kernel is what
makes containers small and quick to start, and it is also their weak point: a
kernel bug reachable from inside is a way out
([container escape](/glossary/container-escape)).

| Property                    | Container               | MicroVM on Runtime                                                    |
| --------------------------- | ----------------------- | --------------------------------------------------------------------- |
| Kernel                      | The host's, shared      | Its own                                                               |
| Boundary for untrusted code | System-call filtering   | Hardware virtualization                                               |
| Start                       | Milliseconds to seconds | 221 ms server-side to a first Python result, median |
| Runs Docker inside          | Only with privileges    | Yes, as a normal user with `sudo`                                     |

## When a container is enough

For your own trusted code, a container is the right tool. For code an agent
wrote, a user pasted or a package installed, a separate kernel is the safer
default, which is why agent sandboxes moved to microVMs.

## Related

- [Docker vs a virtual machine](/compare/docker-vs-virtual-machine)
- [MicroVM vs container](/compare/microvm-vs-container)
- [What is a container escape?](/glossary/container-escape)
- [What is gVisor?](/glossary/gvisor)

## Sources

Checked 27 September 2026.

- [What is a container?, Docker](https://docs.docker.com/get-started/docker-concepts/the-basics/what-is-a-container/)