Add container detection utility to hermes_constants

Extract `is_container()` detection logic from scattered locations
(`config.py`, `voice_mode.py`) into a centralized, cached function in
`hermes_constants.py`. This follows the same pattern as `is_wsl()` and
`is_termux()` — checking `/.dockerenv`, `/run/.containerenv`, and cgroup
markers.

Update gateway status detection (`status.py`, `dump.py`) to use the new
utility and handle Docker/Podman differently from systemd-based systems.
Update setup guidance (`setup.py`) to show Docker restart instructions
when running in a container.

Add Dockerfile.test for CI integration testing and spec.md as a Python
module taste guide for contributors.

tests/hermes_cli/test_gateway_service.py

@@ -401,6 +401,14 @@ class TestGatewayServiceDetection:
 
         assert gateway_cli.supports_systemd_services() is False
 
+    def test_supports_systemd_services_returns_true_when_systemctl_present(self, monkeypatch):
+        monkeypatch.setattr(gateway_cli, "is_linux", lambda: True)
+        monkeypatch.setattr(gateway_cli, "is_termux", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_wsl", lambda: False)
+        monkeypatch.setattr(gateway_cli.shutil, "which", lambda name: "/usr/bin/systemctl")
+
+        assert gateway_cli.supports_systemd_services() is True
+
     def test_is_service_running_checks_system_scope_when_user_scope_is_inactive(self, monkeypatch):
         user_unit = SimpleNamespace(exists=lambda: True)
         system_unit = SimpleNamespace(exists=lambda: True)
@@ -1025,3 +1033,91 @@ class TestSystemUnitPathRemapping:
         # Target user paths should be present
         assert "/home/alice" in unit
         assert "WorkingDirectory=/home/alice/.hermes/hermes-agent" in unit
+
+
+class TestDockerAwareGateway:
+    """Tests for Docker container awareness in gateway commands."""
+
+    def test_run_systemctl_raises_runtimeerror_when_missing(self, monkeypatch):
+        """_run_systemctl raises RuntimeError with container guidance when systemctl is absent."""
+        import pytest
+
+        def fake_run(cmd, **kwargs):
+            raise FileNotFoundError("systemctl")
+
+        monkeypatch.setattr(gateway_cli.subprocess, "run", fake_run)
+
+        with pytest.raises(RuntimeError, match="systemctl is not available"):
+            gateway_cli._run_systemctl(["start", "hermes-gateway"])
+
+    def test_run_systemctl_passes_through_on_success(self, monkeypatch):
+        """_run_systemctl delegates to subprocess.run when systemctl exists."""
+        calls = []
+
+        def fake_run(cmd, **kwargs):
+            calls.append(cmd)
+            return SimpleNamespace(returncode=0, stdout="", stderr="")
+
+        monkeypatch.setattr(gateway_cli.subprocess, "run", fake_run)
+
+        result = gateway_cli._run_systemctl(["status", "hermes-gateway"])
+        assert result.returncode == 0
+        assert len(calls) == 1
+        assert "status" in calls[0]
+
+    def test_install_in_container_prints_docker_guidance(self, monkeypatch, capsys):
+        """'hermes gateway install' inside Docker exits 0 with container guidance."""
+        import pytest
+
+        monkeypatch.setattr(gateway_cli, "is_managed", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_termux", lambda: False)
+        monkeypatch.setattr(gateway_cli, "supports_systemd_services", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_macos", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_wsl", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_container", lambda: True)
+
+        args = SimpleNamespace(gateway_command="install", force=False, system=False, run_as_user=None)
+        with pytest.raises(SystemExit) as exc_info:
+            gateway_cli.gateway_command(args)
+
+        assert exc_info.value.code == 0
+        out = capsys.readouterr().out
+        assert "Docker" in out or "docker" in out
+        assert "restart" in out.lower()
+
+    def test_uninstall_in_container_prints_docker_guidance(self, monkeypatch, capsys):
+        """'hermes gateway uninstall' inside Docker exits 0 with container guidance."""
+        import pytest
+
+        monkeypatch.setattr(gateway_cli, "is_managed", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_termux", lambda: False)
+        monkeypatch.setattr(gateway_cli, "supports_systemd_services", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_macos", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_container", lambda: True)
+
+        args = SimpleNamespace(gateway_command="uninstall", system=False)
+        with pytest.raises(SystemExit) as exc_info:
+            gateway_cli.gateway_command(args)
+
+        assert exc_info.value.code == 0
+        out = capsys.readouterr().out
+        assert "docker" in out.lower()
+
+    def test_start_in_container_prints_docker_guidance(self, monkeypatch, capsys):
+        """'hermes gateway start' inside Docker exits 0 with container guidance."""
+        import pytest
+
+        monkeypatch.setattr(gateway_cli, "is_termux", lambda: False)
+        monkeypatch.setattr(gateway_cli, "supports_systemd_services", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_macos", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_wsl", lambda: False)
+        monkeypatch.setattr(gateway_cli, "is_container", lambda: True)
+
+        args = SimpleNamespace(gateway_command="start", system=False)
+        with pytest.raises(SystemExit) as exc_info:
+            gateway_cli.gateway_command(args)
+
+        assert exc_info.value.code == 0
+        out = capsys.readouterr().out
+        assert "docker" in out.lower()
+        assert "hermes gateway run" in out