sysext: Add debugging commands

When unsure what files an image ships or why the image is not compatible
it can help to list the image contents or show the debug output from the
matching.

docs/provisioning/sysext/_index.md

@@ -184,4 +184,19 @@ systemd:
 This configuration will enable the `systemd-sysupdate.timer` unit that will check every 2-6 hours for a new Docker sysext image available from the latest release of [`sysext-bakery`][sysext-bakery].
 Use `arm64` instead of `x86-64` for arm64 machines.
 
+## Debugging
+
+You can list the contents of a systemd-sysext image like this (assuming an image without dm-verity):
+
+```
+sudo unshare -m sh -c "mount docker-compose-2.18.1.raw /tmp && cd /tmp && find usr"
+# If you use systemd-sysext for extending "/opt", use "find opt" as last command
+```
+
+To get more information about found incompatibilities during merging, enable the debug output:
+
+```
+sudo SYSTEMD_LOG_LEVEL=debug systemd-sysext refresh
+```
+
 [sysext-bakery]: https://github.com/flatcar/sysext-bakery