結論

docker(appium/appium)で立てたappium serverを通してwebviewの項目にswitchするとエラーになる
対処するにはapium server内にchromedriverをインストールしておく必要がある

エラー：appium-serverにchromedriverをイントールしていない状態

# pytest -s test_main.py 
================================================== test session starts ===================================================
platform linux -- Python 3.6.9, pytest-5.4.1, py-1.8.1, pluggy-0.13.1
rootdir: /root/project/appium-script
plugins: forked-1.1.3, xdist-1.31.0
collected 1 item                                                                                                         

    def test_ehlo(self):
  
        self.driver = webdriver.Remote('http://yourappium-server-address:4723/wd/hub', self.desired_caps)

        webview = self.driver.contexts[-1]
>       self.driver.switch_to.context(webview)

test_main.py:41: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/usr/local/lib/python3.6/dist-packages/appium/webdriver/switch_to.py:30: in context
    self._driver.execute(MobileCommand.SWITCH_TO_CONTEXT, {'name': context_name})
/usr/local/lib/python3.6/dist-packages/selenium/webdriver/remote/webdriver.py:321: in execute
    self.error_handler.check_response(response)
/usr/local/lib/python3.6/dist-packages/appium/webdriver/errorhandler.py:29: in check_response
    raise wde
/usr/local/lib/python3.6/dist-packages/appium/webdriver/errorhandler.py:24: in check_response
    super(MobileErrorHandler, self).check_response(response)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <appium.webdriver.errorhandler.MobileErrorHandler object at 0x7fcf44ee8908>
response = {'status': 500, 'value': '{"value":{"error":"unknown error","message":"An unknown server-side error occurred while pro...    at asyncHandler (/usr/lib/node_modules/appium/node_modules/appium-base-driver/lib/protocol/protocol.js:388:37)"}}'}

    def check_response(self, response):
        """
        Checks that a JSON response from the WebDriver does not have an error.
    
        :Args:
         - response - The JSON response from the WebDriver server as a dictionary
           object.
    
        :Raises: If the response contains an error message.
        """
        status = response.get('status', None)
        if status is None or status == ErrorCode.SUCCESS:
            return
        value = None
        message = response.get("message", "")
        screen = response.get("screen", "")
        stacktrace = None
        if isinstance(status, int):
            value_json = response.get('value', None)
            if value_json and isinstance(value_json, basestring):
                import json
                try:
                    value = json.loads(value_json)
                    if len(value.keys()) == 1:
                        value = value['value']
                    status = value.get('error', None)
                    if status is None:
                        status = value["status"]
                        message = value["value"]
                        if not isinstance(message, basestring):
                            value = message
                            message = message.get('message')
                    else:
                        message = value.get('message', None)
                except ValueError:
                    pass
    
        exception_class = ErrorInResponseException
        if status in ErrorCode.NO_SUCH_ELEMENT:
            exception_class = NoSuchElementException
        elif status in ErrorCode.NO_SUCH_FRAME:
            exception_class = NoSuchFrameException
        elif status in ErrorCode.NO_SUCH_WINDOW:
            exception_class = NoSuchWindowException
        elif status in ErrorCode.STALE_ELEMENT_REFERENCE:
            exception_class = StaleElementReferenceException
        elif status in ErrorCode.ELEMENT_NOT_VISIBLE:
            exception_class = ElementNotVisibleException
        elif status in ErrorCode.INVALID_ELEMENT_STATE:
            exception_class = InvalidElementStateException
        elif status in ErrorCode.INVALID_SELECTOR \
                or status in ErrorCode.INVALID_XPATH_SELECTOR \
                or status in ErrorCode.INVALID_XPATH_SELECTOR_RETURN_TYPER:
            exception_class = InvalidSelectorException
        elif status in ErrorCode.ELEMENT_IS_NOT_SELECTABLE:
            exception_class = ElementNotSelectableException
        elif status in ErrorCode.ELEMENT_NOT_INTERACTABLE:
            exception_class = ElementNotInteractableException
        elif status in ErrorCode.INVALID_COOKIE_DOMAIN:
            exception_class = InvalidCookieDomainException
        elif status in ErrorCode.UNABLE_TO_SET_COOKIE:
            exception_class = UnableToSetCookieException
        elif status in ErrorCode.TIMEOUT:
            exception_class = TimeoutException
        elif status in ErrorCode.SCRIPT_TIMEOUT:
            exception_class = TimeoutException
        elif status in ErrorCode.UNKNOWN_ERROR:
            exception_class = WebDriverException
        elif status in ErrorCode.UNEXPECTED_ALERT_OPEN:
            exception_class = UnexpectedAlertPresentException
        elif status in ErrorCode.NO_ALERT_OPEN:
            exception_class = NoAlertPresentException
        elif status in ErrorCode.IME_NOT_AVAILABLE:
            exception_class = ImeNotAvailableException
        elif status in ErrorCode.IME_ENGINE_ACTIVATION_FAILED:
            exception_class = ImeActivationFailedException
        elif status in ErrorCode.MOVE_TARGET_OUT_OF_BOUNDS:
            exception_class = MoveTargetOutOfBoundsException
        elif status in ErrorCode.JAVASCRIPT_ERROR:
            exception_class = JavascriptException
        elif status in ErrorCode.SESSION_NOT_CREATED:
            exception_class = SessionNotCreatedException
        elif status in ErrorCode.INVALID_ARGUMENT:
            exception_class = InvalidArgumentException
        elif status in ErrorCode.NO_SUCH_COOKIE:
            exception_class = NoSuchCookieException
        elif status in ErrorCode.UNABLE_TO_CAPTURE_SCREEN:
            exception_class = ScreenshotException
        elif status in ErrorCode.ELEMENT_CLICK_INTERCEPTED:
            exception_class = ElementClickInterceptedException
        elif status in ErrorCode.INSECURE_CERTIFICATE:
            exception_class = InsecureCertificateException
        elif status in ErrorCode.INVALID_COORDINATES:
            exception_class = InvalidCoordinatesException
        elif status in ErrorCode.INVALID_SESSION_ID:
            exception_class = InvalidSessionIdException
        elif status in ErrorCode.UNKNOWN_METHOD:
            exception_class = UnknownMethodException
        else:
            exception_class = WebDriverException
        if value == '' or value is None:
            value = response['value']
        if isinstance(value, basestring):
            if exception_class == ErrorInResponseException:
                raise exception_class(response, value)
            raise exception_class(value)
        if message == "" and 'message' in value:
            message = value['message']
    
        screen = None
        if 'screen' in value:
            screen = value['screen']
    
        stacktrace = None
        if 'stackTrace' in value and value['stackTrace']:
            stacktrace = []
            try:
                for frame in value['stackTrace']:
                    line = self._value_or_default(frame, 'lineNumber', '')
                    file = self._value_or_default(frame, 'fileName', '<anonymous>')
                    if line:
                        file = "%s:%s" % (file, line)
                    meth = self._value_or_default(frame, 'methodName', '<anonymous>')
                    if 'className' in frame:
                        meth = "%s.%s" % (frame['className'], meth)
                    msg = "    at %s (%s)"
                    msg = msg % (meth, file)
                    stacktrace.append(msg)
            except TypeError:
                pass
        if exception_class == ErrorInResponseException:
            raise exception_class(response, message)
        elif exception_class == UnexpectedAlertPresentException:
            alert_text = None
            if 'data' in value:
                alert_text = value['data'].get('text')
            elif 'alert' in value:
                alert_text = value['alert'].get('text')
            raise exception_class(message, screen, stacktrace, alert_text)
>       raise exception_class(message, screen, stacktrace)
E       selenium.common.exceptions.WebDriverException: Message: An unknown server-side error occurred while processing the command. Original error: Trying to use a chromedriver binary at the path /usr/bin/chromedriver, but it doesn't exist!

対処：chromedriverを含めた状態でappium-serverを作成(Dockerfile)

FROM    appium/appium

WORKDIR /tmp

RUN     apt-get update && \
        apt-get install -y --no-install-recommends wget unzip

RUN     wget https://chromedriver.storage.googleapis.com/80.0.3987.106/chromedriver_linux64.zip && \
        unzip chromedriver_linux64.zip && \
        mv chromedriver /usr/bin/. && \
        chmod +x /usr/bin/chromedriver

よく考えたら、、、

ホストからchromedriverをマウントして渡せばできたな、、、