FP6IMG_UI selects the postmarketOS UI (plasma-mobile, the default, or
phosh). The package set is the same except that the Phosh image adds
imsd's opt-in imsd-ofono subpackage, the GNOME Calls backend; everything
else in it is UI-agnostic. The archive is named after the UI
(fp6-img-phosh.tar.xz), build-info.txt records it, and the workflow runs
the Phosh build as a second step of the same job with FP6IMG_KEEP_DIST=1
so both archives land in one release. Both knobs are carried across the
su into the build user.
The Phosh variant needs imsd 0.3.8 in the registry: that release ships
the preset that enables imsd-ofonod and keeps GNOME Calls alive across
imsd restarts.