A fork from robbertkl/roundcube

Roundcube Docker container:
- Made for robbertkl/docker-mail, but can be used separately
- It differs from alunduil/roundcube in a number of ways:
- Built from stable Roundcube releases, instead of a random master snapshot
- Uses NGINX instead of Apache
- No SSL support (use a separate reverse proxy container if you need SSL)
- Hardcoded to sqlite (no need to configure a database)
- Defines a volume for database persistence
- Includes an additional plugin for carddav
- Some default configuration, like enabled plugins and connection settings