Simplify deployment

[mgruner]

• Improve handling of default values
  • Move them to default interpolation values in the YAML file. This makes it possible to deploy without any .env file that needs to be maintained (if the default values are good enough, e.g. for testing), while being backwards compatible.
• Improve handling of NGINX port configuration
  • Make proper use of the existing NGINX_PORT variable (for expose) and add a new NGINX_EXPOSE_PORT variable that controls the exposed port of the application.
• Improve customization handling
  • nginx port settings were moved to the main configuration file, so that the docker-compose.override.yml could be removed from the git repository. This allows for local customizations with that file in the same way as before, without having conflicts from git or other updates by file overwrites.
• Improve backup handling
  • The backup script was moved to the zammad image with 6.3.1-38. Use that instead via the docker entrypoint, so that no separate custom mount is necessary.

This greatly simplifies the deployment e.g. via Portainer, where now a deployment directly from git is possible just by entering the project URL, no ENVs have to be specified. Customizations via ENV and custom YAML files is possible just like before.