Revert "Remove CI"

This also modifies the gitlab-ci script to include gnome-nightly for the
required SDK. It also tags the flatpak build (so it runs on a suitable
server).

This reverts commit 2c102f97.

.gitlab-ci.yml

+# SPDX-FileCopyrightText: 2018-2023 Bruce Cowan <bruce@bcowan.me.uk>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+flatpak:
+  image: alpine:latest
+  stage: build
+  variables:
+    MANIFEST_PATH: "uk.me.bcowan.Rugby.yml"
+    RUNTIME_REPO: "https://flathub.org/repo/flathub.flatpakrepo"
+    FLATPAK_MODULE: "rugby"
+    BUNDLE: "rugby.flatpak"
+    APP_ID: "uk.me.bcowan.Rugby"
+
+  script:
+  - apk update && apk add flatpak-builder
+  - 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
+  - flatpak install -y gnome-nightly org.gnome.Sdk master
+  - flatpak-builder --stop-at=${FLATPAK_MODULE} app ${MANIFEST_PATH}
+  - flatpak build app meson --prefix=/app ${MESON_ARGS} _build
+  - flatpak build app ninja -C _build install
+  - flatpak-builder --finish-only --repo=repo app ${MANIFEST_PATH}
+  - flatpak build-bundle repo ${BUNDLE} --runtime-repo=${RUNTIME_REPO} ${APP_ID}
+  artifacts:
+    paths:
+    - ${BUNDLE}
+    - _build/meson-logs/meson-log.txt
+  cache:
+    paths:
+      - .flatpak-builder/cache
+  tags:
+    - flatpak