Move Docker images to a Debian slim base

[damien-carcel]

• Use Debian slim as a base for PHP and FPM images in the API
  • Use debian:buster-slim as a base
  • Use Sury packages to install PHP and FPM
  • Build only one image with FPM, but with bash as CMD and launch FPM through docker-compose
• Use Debian slim based image for node services too in the client app