fedora-remote: minimal headless GNOME Remote Desktop host with Kanidm

New image built from upstream base-atomic (no desktop), adding only gnome-shell
+ gdm + gnome-remote-desktop + pipewire/portal + foot + Flatpak + Kanidm.
recommends=false to stay lean. Configures GNOME Remote Desktop headless remote
login on first boot (TLS + credentials + enable). Firefox preinstalled as a
per-user Flatpak. Add it to the CI matrix and prune both images.

images/fedora-remote/build.conf

@@ -0,0 +1,10 @@
+# Build configuration for images/fedora-remote. Sourced by build.sh.
+
+BUILD_MODE=upstream
+
+UPSTREAM_REPO="https://gitlab.com/fedora/ostree/ci-test.git"
+
+# Stable only; this is a small remote-access host.
+declare -A UPSTREAM_REFS=(
+  [44]=f44
+)