plugin app manager docker: use default emails for weblate: when `email_from` was not set in `sat.conf`, no value was set for `WEBLATE_SERVER_EMAIL` and `WEBLATE_DEFAULT_FROM_EMAIL`. These environment variables are needed for Weblate to be launched, so default value are now used (the same as the one from official Weblate Docker image).

version: "3.6"
services:

  sat:
    image: salutatoi/sat
    build: backend

  libervia:
    image: salutatoi/libervia
    build: libervia
    depends_on:
      - sat