core (main): add a mechanism to have a shared temp directory: this directory may be used to share files between backend and frontends. Normally, an os-dependent temporary directory is created for that, but if this option is set, the directory will be created in <local_dir>/<cache_dir>, which may be useful in some use case (e.g. containerized frontends and backend).

version: "3.6"
services:

  prosody:
    image: libervia/prosody:demo
    build: prosody-demo

  backend:
    image: libervia/backend:dev-demo
    build: backend-dev-demo

  web:
    image: libervia/web:dev-demo
    build: libervia-web-dev-demo