fix(docker): add explicit env allowlist for container credentials (#1436)

Docker terminal sessions are secret-dark by default. This adds
terminal.docker_forward_env as an explicit allowlist for env vars
that may be forwarded into Docker containers.

Values resolve from the current shell first, then fall back to
~/.hermes/.env. Only variables the user explicitly lists are
forwarded — nothing is auto-exposed.

Cherry-picked from PR #1449 by @teknium1, conflict-resolved onto
current main.

Fixes #1436
Supersedes #1439

tests/tools/test_parse_env_var.py

@@ -30,6 +30,28 @@ class TestParseEnvVar:
             result = _parse_env_var("TERMINAL_DOCKER_VOLUMES", "[]", json.loads, "valid JSON")
             assert result == ["/host:/container"]
 
+    def test_get_env_config_parses_docker_forward_env_json(self):
+        with patch.dict("os.environ", {
+            "TERMINAL_ENV": "docker",
+            "TERMINAL_DOCKER_FORWARD_ENV": '["GITHUB_TOKEN", "NPM_TOKEN"]',
+        }, clear=False):
+            config = _tt_mod._get_env_config()
+            assert config["docker_forward_env"] == ["GITHUB_TOKEN", "NPM_TOKEN"]
+
+    def test_create_environment_passes_docker_forward_env(self):
+        fake_env = object()
+        with patch.object(_tt_mod, "_DockerEnvironment", return_value=fake_env) as mock_docker:
+            result = _tt_mod._create_environment(
+                "docker",
+                image="python:3.11",
+                cwd="/root",
+                timeout=180,
+                container_config={"docker_forward_env": ["GITHUB_TOKEN"]},
+            )
+
+        assert result is fake_env
+        assert mock_docker.call_args.kwargs["forward_env"] == ["GITHUB_TOKEN"]
+
     def test_falls_back_to_default(self):
         with patch.dict("os.environ", {}, clear=False):
             # Remove the var if it exists, rely on default