fix(docker): make host Docker socket opt-in (#4902)

* fix(docker): make host socket compose opt-in

* fix(cookbook): gate container Docker access

* fix(docker): gate socket group setup on opt-in

* fix(cookbook): gate generated docker exec serve commands

* fix(cookbook): narrow generated docker exec forms

src/host_docker_access.py

@@ -0,0 +1,62 @@
+"""Policy checks for explicit host Docker access from a container."""
+
+import os
+import stat
+from collections.abc import Mapping
+
+
+HOST_DOCKER_ENV_VAR = "ODYSSEUS_ENABLE_HOST_DOCKER"
+HOST_DOCKER_SOCKET_PATH = "/var/run/docker.sock"
+
+HOST_DOCKER_ACCESS_HINT = (
+    "Local Docker daemon access is disabled inside the Odysseus container; a "
+    "Docker CLI alone is not enough. Default Docker Compose intentionally does "
+    "not mount the host Docker socket. Raw socket access is high-trust and can "
+    "grant broad control over the host Docker daemon. If you accept that risk, "
+    "enable docker/host-docker.yml. Remote server Docker workflows over SSH "
+    "remain preferred."
+)
+
+
+def running_in_container(
+    dockerenv_path: str = "/.dockerenv",
+    cgroup_path: str = "/proc/1/cgroup",
+) -> bool:
+    if os.path.exists(dockerenv_path):
+        return True
+    try:
+        with open(cgroup_path, "r", encoding="utf-8") as handle:
+            contents = handle.read()
+    except OSError:
+        return False
+    return any(token in contents for token in ("docker", "containerd", "kubepods"))
+
+
+def host_docker_access_enabled(
+    socket_path: str = HOST_DOCKER_SOCKET_PATH,
+    *,
+    environ: Mapping[str, str] | None = None,
+) -> bool:
+    env = os.environ if environ is None else environ
+    if env.get(HOST_DOCKER_ENV_VAR, "").strip().lower() != "true":
+        return False
+    try:
+        mode = os.stat(socket_path).st_mode
+    except OSError:
+        return False
+    return stat.S_ISSOCK(mode)
+
+
+def local_docker_available(
+    *,
+    cli_available: bool,
+    in_container: bool | None = None,
+    environ: Mapping[str, str] | None = None,
+    socket_path: str = HOST_DOCKER_SOCKET_PATH,
+) -> bool:
+    if not cli_available:
+        return False
+    containerized = running_in_container() if in_container is None else in_container
+    if not containerized:
+        return True
+    return host_docker_access_enabled(socket_path, environ=environ)