docker: dockers images, first draft: This patch introduce new docker images, replacing the unmaintained ones which were in sat_docs. Images + docker-compose files are provided to use SàT backend and Libervia, but also to integrate and configure Prosody and SàT PubSub to run end-to-end tests.

version: "3.6"
services:

  sat:
    image: salutatoi/sat
    build: backend

  libervia:
    build: libervia
    depends_on:
      - sat