fix(security): mount host Docker socket only in aio (DooD) sandbox mode (#3517)

* fix(security): mount host Docker socket only in aio (DooD) sandbox mode

The default Compose stack mounted /var/run/docker.sock read-write into the
root gateway container in every sandbox mode, including the default `local`
mode that never uses it -- an unnecessary host-escape surface (DooD =
root-equivalent host control). deploy.sh already gated the socket *check* on
sandbox_mode != local, but the Compose files mounted it unconditionally.

Move the socket mount to an opt-in docker/docker-compose.dood.yaml overlay
that deploy.sh / docker.sh append only when detect_sandbox_mode() returns
`aio`. Default (local) and provisioner/Kubernetes modes no longer expose the
host daemon. Tighten the socket existence check from != local to == aio.
Document the DooD threat model in SECURITY.md.

Reported by @greatmengqi.

* refactor(docker): address review on socket-hardening PR

- docker.sh: use absolute path for the dood overlay (match deploy.sh, drop cwd dependency)
- deploy.sh: drop now-dead DEER_FLOW_DOCKER_SOCKET exports in down/build paths
- docker-compose.yaml: fix stale header comment to point at the overlay

Addresses codex + reviewer feedback on #3517.

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

docker/docker-compose-dev.yaml

@@ -146,8 +146,10 @@ services:
       # On macOS/Docker Desktop, uv may fail to create symlinks inside shared
       # host directories, which causes startup-time `uv sync` to crash.
       - gateway-uv-cache:/root/.cache/uv
-      # DooD: AioSandboxProvider runs inside the Gateway process.
-      - /var/run/docker.sock:/var/run/docker.sock
+      # DooD: the host Docker socket is NOT mounted by default. It is added only
+      # for aio (pure-DooD) sandbox mode via the opt-in docker-compose.dood.yaml
+      # overlay (appended by scripts/docker.sh). See SECURITY.md.
+      
       # CLI auth dirs (Claude Code / Codex) are NOT mounted by default: they
       # expose the entire ~/.claude and ~/.codex (history, projects, global
       # config, credentials) into the container. Mount them only when you use