Ensure enough memory for Composer

Summary:
Resolve a composer.json for an old Laravel 5.2 code base now consumes
1.8 GiB of RAM.

It seems reasonable to allow this container specialized to run PHP
CLI applications to let the application to consume 2 GiB of RAM.

Fixes T1638.

Test Plan: composer install

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1638

Differential Revision: https://devcentral.nasqueron.org/D2367