A Symfony 2.1 app to be more specific. The great feature that Symfony 2 was introducing, compared to Symfony 1, was the notion of splitting code into bundles, and building apps around this concept.
PHP 7 support in Symfony starts at Symfony 2.3, so we had to migrate Symfony to at least 2.3.- We had to support both versions of PHP (5.6 and 7.1) so that we could rollback the PHP version if something went wrong with PHP once in production. As we had to ensure that our code would be compatible with two versions of PHP, the easier way to proceed was to use Docker, and simply change the version of the PHP image we would be using.
And as we had 64 dependencies to work on, we quickly decided that it was impossible to migrate every single one by hand.