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/how-tos/print-configuration.md

@@ -0,0 +1,27 @@
+---
+title: Show loaded configuration
+layout: default
+parent: How Tos
+nav_order: 8
+---
+
+# Print loaded configuration
+
+You can print the configuration that `docker-volume-backup` has picked up without running a backup:
+
+```console
+docker exec <container_ref> backup print-config
+```
+
+If configuration sourcing fails, the error is printed to stdout to aid debugging.
+
+If you want to test a one-off value, pass it directly:
+
+```console
+docker exec -e BACKUP_SOURCES=/backup -e NOTIFICATION_URLS=stdout:// <container_ref> backup print-config
+```
+{: .note }
+Output includes secrets exactly as loaded.
+
+{: .warning }
+This feature is still in development and might change in future releases.