fix(gateway): harden Docker/container gateway pathway

Centralize container detection in hermes_constants.is_container() with
process-lifetime caching, matching existing is_wsl()/is_termux() patterns.
Dedup _is_inside_container() in config.py to delegate to the new function.

Add _run_systemctl() wrapper that converts FileNotFoundError to RuntimeError
for defense-in-depth — all 10 bare subprocess.run(_systemctl_cmd(...)) call
sites now route through it.

Make supports_systemd_services() return False in containers and when
systemctl binary is absent (shutil.which check).

Add Docker-specific guidance in gateway_command() for install/uninstall/start
subcommands — exit 0 with helpful instructions instead of crashing.

Make 'hermes status' show 'Manager: docker (foreground)' and 'hermes dump'
show 'running (docker, pid N)' inside containers.

Fix setup_gateway() to use supports_systemd instead of _is_linux for all
systemd-related branches, and show Docker restart policy instructions in
containers.

Replace inline /.dockerenv check in voice_mode.py with is_container().

Fixes #7420

Co-authored-by: teknium1 <teknium1@users.noreply.github.com>

hermes_constants.py

@@ -189,6 +189,37 @@ def is_wsl() -> bool:
     return _wsl_detected
 
 
+_container_detected: bool | None = None
+
+
+def is_container() -> bool:
+    """Return True when running inside a Docker/Podman container.
+
+    Checks ``/.dockerenv`` (Docker), ``/run/.containerenv`` (Podman),
+    and ``/proc/1/cgroup`` for container runtime markers.  Result is
+    cached for the process lifetime.  Import-safe — no heavy deps.
+    """
+    global _container_detected
+    if _container_detected is not None:
+        return _container_detected
+    if os.path.exists("/.dockerenv"):
+        _container_detected = True
+        return True
+    if os.path.exists("/run/.containerenv"):
+        _container_detected = True
+        return True
+    try:
+        with open("/proc/1/cgroup", "r") as f:
+            cgroup = f.read()
+            if "docker" in cgroup or "podman" in cgroup or "/lxc/" in cgroup:
+                _container_detected = True
+                return True
+    except OSError:
+        pass
+    _container_detected = False
+    return False
+
+
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