feat(docker): add Docker-in-Docker feature for testcontainers

[arv]

What

Adds a new docker dev container feature that gives containers a self-contained Docker daemon, mirroring how agents wraps the official claude-code/github-cli features.

It dependsOn the official docker-in-docker feature (engine + in-container daemon + adds the remote user to the docker group) and serves as the single, centrally-pinned entry point for Docker across rocicorp repos.

Why

Tooling that shells out to Docker fails in the dev container today. Running pnpm test in packages/zero-cache errors with:

Error: Could not find a working container runtime strategy
 ❯ PostgreSqlContainer.start .../@testcontainers/postgresql/...
 ❯ Object.setup test/pg-container-setup.ts:5:23

The pg test suite spins up Postgres via testcontainers, which needs a Docker daemon. The dev container ships none (no docker binary, no socket).

Why docker-in-docker (not docker-outside-of-docker)

testcontainers relies on bind mounts and container-to-container networking. The host-socket approach breaks bind-mount path translation and isn't available in every environment (Codespaces, CI). A self-contained daemon works everywhere.

Usage

"features": {
  "ghcr.io/rocicorp/devcontainer-features/docker:1": {}
}

Follow-up

Once this merges and the release workflow publishes ghcr.io/rocicorp/devcontainer-features/docker, the consumer change in rocicorp/mono (.devcontainer/devcontainer.json references the feature) can be merged/rebuilt.

🤖 Generated with Claude Code