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).

[DEFAULT]
media_dir = /src/sat_media

[jp]
# FIXME: default "auto" value cause trouble
background = dark