feat: optional read-only root filesystem for run pods in the Kubernetes supervisor #5078

Is your feature request related to a problem? Please describe.

Self-hosters running Trigger.dev on Kubernetes with Pod Security / policy engines (Kyverno, OPA Gatekeeper, CIS benchmarks, some compliance scanners) are often required to run containers with readOnlyRootFilesystem: true. The supervisor's KUBERNETES_RUNNER_SECURITY_CONTEXT (off/baseline/restricted) doesn't set it, and no Pod Security Standards level requires it, so there is currently no way to get it on run pods without patching the supervisor.

Describe the solution you'd like to see

Add two opt-in supervisor env vars:

• KUBERNETES_RUNNER_READ_ONLY_ROOT_FILESYSTEM (boolean, default false). When true, sets securityContext.readOnlyRootFilesystem: true on the run-controller container. It composes with any KUBERNETES_RUNNER_SECURITY_CONTEXT level, including off.
• KUBERNETES_RUNNER_TMP_VOLUME_SIZE_LIMIT (default 256Mi, blank = no per-volume cap). When read-only is enabled, an emptyDir is mounted at /tmp so tasks keep a writable os.tmpdir().

The runner runtime itself doesn't write to disk (IPC over socketpair, logs to stdout, OTLP over HTTP), so a single /tmp volume is sufficient. Default behavior is unchanged.

Describe alternate solutions

• A mutating admission webhook injecting readOnlyRootFilesystem and a /tmp volume. This works but adds cluster-side machinery for something the supervisor can do natively.
• Maintaining a fork of the supervisor. This is what we do today and what we'd like to stop doing.
• Making KUBERNETES_RUNNER_SECURITY_CONTEXT=restricted imply the flag. This would break existing users, since restricted doesn't imply a read-only root.

Additional information

We already run this in a private fork (including unit tests for runnerSecurityContext and the new runnerTmpVolume helper). The change touches apps/supervisor/src/env.ts, workloadManager/kubernetes.ts and workloadManager/kubernetesPodSpec.ts, and is roughly 60 lines of source. If this direction is acceptable, we'd be happy to open a PR once vouched, so we can drop our fork.