test: establish Docker Compose support and add smoke coverage

Description

Establish whether the checked-in Docker Compose deployment is a supported OpenShell configuration. If it is supported, add automated Compose-based smoke coverage that verifies the user-facing path from starting the gateway through creating a Docker-backed sandbox.

Context

The Compose deployment currently lacks end-to-end coverage. Issue #2891 reports that its gateway cannot create sandboxes because gateway JWT material is not configured. This work should make the supported-configuration decision explicit and prevent compatible deployment regressions from escaping without a runnable check.

Definition of Done

• Maintainers record whether deploy/docker/docker-compose.yml is a supported configuration, including the intended support boundary.
• If supported, automated coverage starts the Compose deployment with mutually compatible gateway and CLI artifacts and creates a Docker-backed sandbox.
• The smoke test checks the externally documented readiness/health behavior that users are expected to rely on.
• The test exercises the sandbox authentication path, including the configuration required to prevent #2891.
• If Compose is not supported, documentation and the deployment artifact clearly communicate the unsupported or experimental status instead of implying a maintained quickstart.

Start by inspecting deploy/docker/docker-compose.yml and the failure described in issue #2891, then identify the documented readiness and health behavior plus the gateway authentication configuration. Determine with maintainers whether Compose is supported; if so, add runnable smoke coverage for startup and Docker-backed sandbox creation, otherwise update the deployment artifact and documentation to state its status.