Add "print-config" subcommand (#698)

* Add "show-config" subcommand

* Add integration tests

* Handle wrong additional args

* Support "BackupFilenameExpand" option

* Rename to print-config

* Remove config nil check as it will never be nil

* Rework print config to work with config.resolve()

* Use defer unset()

* Add warning about feature not being stable yet

docs/reference/index.md

@@ -19,6 +19,7 @@ You can work around this by either updating `docker-compose` or unquoting your c
 
 You can populate below template according to your requirements and use it as your `env_file`.
 The values for each key currently match its default.
+If you need to confirm what the container actually loaded, see [Show loaded configuration](../how-tos/print-configuration.md).
 
 {% raw %}
 ```