Set allowed email domains in .env file for docker compose

[JPromi]

Set allowed E-Mail Domains in .env file if using docker compose

[mikonse]

Hi,
that should already be possible by setting the following environment variable.

All settings are essentially supported via environment variables, subkeys are delimited by __ and complex types like lists are parsed as json.

ABRECHNUNG_REGISTRATION__VALID_EMAIL_DOMAINS=["domain1","domain2","domain3"]