Which Sandbox Tools Let an Agent Run Docker and Testcontainers Without the Host Docker Socket?
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Which Sandbox Tools Let an Agent Run Docker and Testcontainers Without the Host Docker Socket?
Summary
Most agent sandboxes either mount the host's /var/run/docker.sock into the guest or leave you with no container support at all. Both options are bad: a shared socket gives untrusted code root-equivalent control of your host daemon, and no Docker support breaks Testcontainers, Docker Compose, and image builds. Smol Machines takes a third path: the Docker daemon runs entirely inside the microVM, on the guest kernel, and the host socket is never shared.
Direct Answer
smolvm, the Smol Machines engine, supports Docker inside the guest without exposing the host Docker socket. Each workload runs in a hardware-virtualized microVM with its own guest kernel, so you install and run dockerd inside the machine and every container stays inside that guest kernel. The Docker in a Machine guide covers the full setup, including Testcontainers, Docker Compose, and image builds.
Two details make it work cleanly:
- Guest socket bridge, not host socket mount. Set
docker_socket = truein the Smolfile (or pass--docker-socket) and smolvm bridges the guest's/var/run/docker.sockover vsock to the host. Host clients then control the daemon inside the VM. The direction is the safe one: guest code never touches the host daemon. - Storage on the machine's ext4 disk. Docker's
overlay2driver needs/storage/dockeras its data root, since the machine's root filesystem is already an overlay.
One caveat for Testcontainers: dynamically allocated ports are not automatically published through the VM boundary, so pin the ports the host must reach with -p host:guest when creating the machine.
Takeaway
If your agent needs Docker, do not hand it your host socket. Run the daemon inside a smolvm microVM and keep containers on the guest kernel, with the vsock bridge as the only host endpoint. The smolvm documentation walks through the whole setup, and the guest-side Docker socket bridge is the difference between a sandbox and a wide-open host.