Get composer build working on GitLab CI

See: https://www.drupal.org/project/drupalci_environments/issues/3387737#comm... 📌 Split PHP image into php(cli/apache) and yarn(node/nightwatch) Needs review

Also andypost in Slack:

@ptmkenny
in case of bundled PHP extensions you need to rebuild PHP+mod_php itself, so better to use own image ATM and let's brainstorm in 3387737 to build sodium and others as shared extensions (--with-sodium=shared as dev-build image for example