Document build pipeline setup to install PHP and Composer dependencies

[eliot-akira]

Document how a plugin can add this to its workflow script as needed for external Composer dependencies.

Related:

• Installing Composer dependencies roller#12
• Install PHP and Composer dependencies pipeline#5

Here's an example GitHub Action workflow.

# Install external deps, generate autoload and lock file
# Configure workspace as safe for Git to solve: https://github.com/composer/composer/issues/12221
- name: Install PHP dependencies (Composer)
  uses: php-actions/composer@v6
  with:
    php_version: '8.2'
  run: |
    git config --global --add safe.directory /app
    composer install --no-dev --prefer-dist --no-interaction --no-progress --optimize-autoloader