PHP 7.1 on Ubuntu with an attempt to load the "DOMDocument" class from the global namespace

Question

I am stuck:

Class NotFoundException in line XmlUtils.php 52: Attempting to load the class "DOMDocument" from the global namespace. Did you forget the expression "use"? in XmlUtils.php (line 52)

I am running the Symfony LTS 2.8 application in a docker container with Apache / 2.4.18 (Ubuntu) and PHP 7.1.9-1 + ubuntu16.04.1 + deb.sury.org + 1 (cli) (built: Sep 2 2017 05:56: 43) (NTS)

and tried:

• clearing cache
• install php7.1-xml and restart
• install php-xml and restart
• install php7.1-dom and restart
• install php7.1-intl and restart
• uninstall xdebug and restart

Installed modules: php -m

PHP Modules
    calendar
    Core
    ctype
    curl
    date
    dom
    exif
    fileinfo
    filter
    ftp
    gd
    gettext
    hash
    iconv
    imap
    intl
    json
    libxml
    mbstring
    mysqli
    mysqlnd
    openssl
    pcntl
    pcre
    PDO
    pdo_myser
    wddx
    xml
    xmlreader
    xmlrpc
    xmlwriter
    xsl
    Zend OPcache
    zip
    zlib

    [Zend Modules]
    Zend OPcache