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).

You'll find install instructions in documentation (see doc/installation.rst).

If you need help, come the SàT official XMPP room: sat@chat.jabberfr.org or join us with a web browser via https://chat.jabberfr.org/converse.js/sat@chat.jabberfr.org