docs: add SECURITY.md with a private reporting route and explicit scope

SECURITY.md

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+# Security policy
+
+## Reporting a vulnerability
+
+**Report privately, not as a public issue.** Use GitHub private vulnerability
+reporting:
+
+> **[Report a vulnerability](https://github.com/schubergphilis/claude-docker/security/advisories/new)**
+
+That opens a draft advisory visible only to you and the maintainers. If you
+cannot use it, open a public issue containing *only* a request for a private
+channel — no details.
+
+Please include the commit or release you tested, the wrapper flags in use
+(`--yolo`, `--aws`, `--gh`, `--ephemeral`, …), which container runtime and host
+OS, and a description of the weakness and its impact. Describe the *class* of
+problem rather than attaching a working exploit, and never include real
+credentials, tokens, or transcript content — a path, a mount mode, or a config
+key is usually enough to make a finding reproducible.
+
+This is a small project maintained on a best-effort basis. There is no bounty
+and no guaranteed response time. Expect an acknowledgement and an initial
+assessment; a fix lands as an ordinary PR, and the advisory is published once a
+fix is available if publication adds anything for users.
+
+## Supported versions
+
+Pre-1.0. Fixes land on `main`; there are no backports to earlier pre-releases.
+
+| Version              | Supported        |
+| -------------------- | ---------------- |
+| `main`               | Yes              |
+| Latest `v0.1.0-rc.*` | Yes, best effort |
+| Earlier pre-releases | No               |
+
+Note that the image is built locally from this repository rather than pulled
+from a registry, so "upgrading" means rebuilding — see
+[Install](README.md#install).
+
+## Scope
+
+In scope: the `run.sh` wrapper, `entrypoint.sh`, the `Dockerfile` and the image
+it produces, the credential opt-in model, the privilege drop and capability set,
+the `gh` auth-proxy sidecar, and the persistent named-volume model.
+
+Out of scope, because they are documented properties rather than defects — read
+the [threat model](README.md#threat-model) before reporting:
+
+- **Full outbound network with no egress filtering.** There is no network
+  policy; a session can reach anything the host can.
+- **Runtime code-fetch.** `npx`, `pnpm dlx`, `uvx`, `tfenv install` and the Go
+  toolchain fetch and execute third-party code on demand, by design.
+- **Workspaces are read-write** unless `--ro` is passed.
+- **Opted-in credentials are readable in-session.** That is what the flag does;
+  the container is a blast-radius reduction, not a secret boundary.
+- **State persists across sessions** in the `claude-code-root` and
+  `claude-code-home` named volumes unless `--ephemeral` is passed.
+- **`--yolo` / `--dangerously-skip-permissions` is a deliberate mode**, not a
+  misconfiguration.
+- **The container is not a full sandbox.** It narrows blast radius compared with
+  running the agent directly on the host; it does not contain a determined
+  attacker who already has code execution.
+
+A finding that a *documented* boundary does not hold as documented is in scope
+and welcome — that is different from reporting the documented boundary itself.
+
+Vulnerabilities in Claude Code, or in the bundled third-party CLIs (`gh`,
+`glab`, `aws`, `terraform`, `uv`, `pnpm`, …), belong upstream with their
+respective projects. Known-vulnerable OS and language packages in the image are
+tracked by the Trivy scans in the repository's CI workflows and, where a finding
+is accepted for a time, recorded with a mandatory expiry in
+[`.trivyignore`](.trivyignore).