Allow *_PASSWORD_FILE env-variables #927
Description
As far as I understood, supporting *_PASSWORD_FILE environment variables seems to be common for docker application that require the configuration of passwords. This seems to be a requirement for supporting the usage of docker secrets and therefore allowing users to keep passwords out of the docker-compose.yml files.
Hence, I see the need for the feature to specifically provide support for the env-vars ICINGADB_DATABASE_PASSWORD_FILE and ICINGADB_REDIS_PASSWORD_FILE as alternatives to the existing *_PASSWORD vars.