QFQ

removed composer with apt-get and installed newest version following this guide: https://stackoverflow.com/a/57584839

1241-0> composer -V
Do not run Composer as root/super user! See https://getcomposer.org/root for details
Continue as root/super user [yes]? 
Composer version 2.0.11 2021-02-24 14:57:23

Pipeline still fails. The problem seems to be that the PHP version on alfred16 is too high:

Your requirements could not be resolved to an installable set of packages.
  Problem 1
    - phpunit/phpunit[6.5.0, ..., 6.5.14] require php ^7.0 -> your php version (8.0.3) does not satisfy that requirement.
    - Root composer.json requires phpunit/phpunit ^6.5 -> satisfiable by phpunit/phpunit[6.5.0, ..., 6.5.14].

PHP version on alfred16 is indeed 8.0.3

1243-1> php --version
PHP 8.0.3 (cli) (built: Mar  5 2021 07:53:39) ( NTS )
Copyright (c) The PHP Group
Zend Engine v4.0.3, Copyright (c) Zend Technologies
    with Zend OPcache v8.0.3, Copyright (c), by Zend Technologies

• purged php8.0 from alfred16
• removed apt repository ondrej-ubuntu-php-xenial.list so it won't happen again.

now php is version 7.2 again