PHP/Bref container customizations

[timkelty]

Allow projects to opt-in to PHP container customization, e.g. to:

• enable additional PHP extensions
• install common binaries for things like:
  • puppeteer
  • headless Chrome (Critical CSS)
  • wkhtmltopdf
  • ffmpeg
  • mjml

[jamesmacwhite]

PHP extensions would be good, I've just hit an issue with the PHP function easter_date() because the calendar extension isn't installed and it wasn't detected until deploying to Craft Cloud.