man: add an FAQ entry about PORTABLE_PREFIXES=

diff --git a/mkosi/resources/man/mkosi.1.md b/mkosi/resources/man/mkosi.1.md
index 7968432233b1d706c99bf06c2afa5c8e43ab19cb..0a44b863bb432058d7004980f06468f8f945b753 100644 (file)
--- a/mkosi/resources/man/mkosi.1.md
+++ b/mkosi/resources/man/mkosi.1.md
@@ -2965,6 +2965,15 @@ Note that the minimum required Python version is 3.9.
   Alternatively, the `z` or `Z` directives for `systemd-tmpfiles` can be used to chown
   various directories and files to their owning user when the system first boots up.
 
+- Why does `portablectl inspect` say my portable service isn't one?
+
+  `portablectl inspect` checks for `PORTABLE_PREFIXES=` in `os-release` and if the key is missing, will fail
+  to recognise a portable service as one. Since there is no good default to set for this key and the
+  generated portable service images will still attach properly, even when the key is not set, mkosi doesn't
+  set one.
+
+  You can set `PORTABLE_PREFIXES=` in the `os-release` file yourself in a postinst script.
+
 # REFERENCES
 * [Primary mkosi git repository on GitHub](https://github.com/systemd/mkosi/)
 * [mkosi — A Tool for Generating OS Images](https://0pointer.net/blog/mkosi-a-tool-for-generating-os-images.html) introductory blog post by Lennart Poettering