Add appstream-compose for CI

.gitlab-ci.yml

-# SPDX-FileCopyrightText: 2018-2023 Bruce Cowan <bruce@bcowan.me.uk>
+# SPDX-FileCopyrightText: 2018-2024 Bruce Cowan <bruce@bcowan.me.uk>
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 flatpak:
@@ -12,7 +12,7 @@ flatpak:
     APP_ID: "uk.me.bcowan.Rugby"
 
   script:
-  - apk update && apk add flatpak-builder
+  - apk update && apk add flatpak-builder appstream-compose
   - flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
   - flatpak remote-add --if-not-exists gnome-nightly https://nightly.gnome.org/gnome-nightly.flatpakrepo
   - flatpak install -y gnome-nightly org.gnome.Platform master