fix: pass search env into Docker task containers (SEARXNG_URL was lost)

Docker tasks do not inherit the worker process environment, they only
get extra_env (-e). SEARXNG_URL in the worker unit EnvironmentFile
therefore never reached codex -> MCP inside the container, so the
auto chain skipped SearXNG and only DuckDuckGo was tried (live e2e:
'all search providers failed: duckduckgo (DNS)').

_docker_task_context now merges any set SEARXNG_URL /
PERPLEXITY_API_KEY / EXA_API_KEY / TAVILY_API_KEY / BRAVE_API_KEY
from the worker environment into extra_env (values never logged;
paid keys follow the same extra_env path as gateway keys).

Tests: 2 new docker-env tests; websearch + tenant isolation = 25
passing; full cloud suite 1038 passing (4 skipped).

cloud/tests/test_websearch_worker.py

@@ -99,6 +99,42 @@ class WorkerWebsearchTestCase(unittest.TestCase):
         self.assertEqual(config.count("[mcp_servers.websearch]"), 1)
 
 
+class DockerEnvTestCase(unittest.TestCase):
+    def setUp(self):
+        self.tmp = tempfile.TemporaryDirectory()
+        self.db_path = os.path.join(self.tmp.name, "control.db")
+        init_db(self.db_path)
+
+    def tearDown(self):
+        self.tmp.cleanup()
+
+    def _docker_context(self):
+        worker = Worker(self.db_path, "/fake/host.py",
+                        env={"XCM_API_KEY": "test-placeholder"},
+                        tenant_isolation=False, egress_proxy_port=None)
+        return worker._docker_task_context({"project_id": "projA"})
+
+    def test_searxng_url_passes_into_container_env(self):
+        from unittest import mock
+        with mock.patch.dict(os.environ,
+                             {"SEARXNG_URL": "http://searx.diante.me"}):
+            ok, _, _, env, err = self._docker_context()
+        self.assertTrue(ok)
+        self.assertIsNone(err)
+        self.assertEqual(env.get("SEARXNG_URL"), "http://searx.diante.me")
+
+    def test_no_search_env_means_no_extra_env(self):
+        from unittest import mock
+        with mock.patch.dict(os.environ, {}, clear=False):
+            for k in ("SEARXNG_URL", "PERPLEXITY_API_KEY", "EXA_API_KEY",
+                      "TAVILY_API_KEY", "BRAVE_API_KEY"):
+                os.environ.pop(k, None)
+            ok, _, _, env, err = self._docker_context()
+        self.assertTrue(ok)
+        self.assertIsNone(err)
+        self.assertIsNone(env)
+
+
 class EgressFilterTestCase(unittest.TestCase):
     def test_search_domains_present_in_basic_form(self):
         lines = open(REPO_ROOT / "deploy" / "egress" /