feat: add environment variables and docs

ref: N25B-352

test/integration/test_config.py

@@ -0,0 +1,32 @@
+from mock import patch, mock
+
+from robot_interface.core.config import Settings
+from robot_interface.endpoints.main_receiver import MainReceiver
+
+
+def test_environment_variables(monkeypatch):
+    """
+    When environment variables are set, creating settings should use these.
+    """
+    monkeypatch.setenv("AGENT__CONTROL_BACKEND_HOST", "some_value_that_should_be_different")
+
+    settings = Settings()
+
+    assert settings.agent_settings.control_backend_host == "some_value_that_should_be_different"
+
+
+@patch("robot_interface.endpoints.main_receiver.settings")
+@patch("robot_interface.endpoints.socket_base.settings")
+def test_create_endpoint_custom_host(base_settings, main_settings):
+    """
+    When a custom host is given in the settings, check that an endpoint's socket connects to it.
+    """
+    fake_context = mock.Mock()
+    fake_socket = mock.Mock()
+    fake_context.socket.return_value = fake_socket
+    base_settings.agent_settings.control_backend_host = "not_localhost"
+    main_settings.agent_settings.main_receiver_port = 9999
+
+    _ = MainReceiver(fake_context)
+
+    fake_socket.connect.assert_called_once_with("tcp://not_localhost:9999")