example.php

<?php 

..
..
$cmd = 'docker build someimage'; 
$process = Process::fromShellCommandline($cmd);
$process->setTimeout(0);
$process->run(function ($type, $buffer) {
    if (Process::ERR === $type) {
        echo 'ERR > '.$buffer;
    } else {
        echo 'OUT > '.$buffer;
    }
});
// executes after the command finishes
if (!$process->isSuccessful()) {
    throw new ProcessFailedException($process);
}
return $process->getOutput();