CI: Update workflow to support container selection

Update the primary CI workflow (renamed from 'main.yml' to 'ci.yml')
to also build for PRs and pushes targeting branches with names of the
form `backports/gnome-*`, and select the corresponding image from
`GSConnect/gsconnect-ci` tagged as `gnome-##`.

This will allow testing branches against GNOME versions other than the
default/current, without requiring separate workflows with near-duplicate
content like the previous `gnome-3-3[68].yml`. (Those are deleted.)

.github/workflows/ci.yml

@@ -0,0 +1,89 @@
+# SPDX-FileCopyrightText: GSConnect Developers https://github.com/GSConnect
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+name: CI
+
+on:
+  push:
+    branches:
+      - main
+      - 'backports/gnome-*'
+  pull_request:
+    branches:
+      - main
+      - 'backports/gnome-*'
+  workflow_dispatch:
+
+jobs:
+  select:
+    runs-on: ubuntu-latest
+    outputs:
+      image_id: ${{ steps.select-image.outputs.image_id }}
+    steps:
+      - id: step1
+        if: ${{ ! startsWith(github.event_name, 'pull_request') }}
+        run: echo "target=${{ github.ref_name }}" >> $GITHUB_OUTPUT
+      - id: step2
+        if: ${{ startsWith(github.event_name, 'pull_request') }}
+        run: echo "target=${{ github.base_ref }}" >> $GITHUB_OUTPUT
+      - id: select-image
+        env:
+          TARGET_BRANCH: ${{ steps.step1.outputs.target || steps.step2.outputs.target }}
+        run: |
+          if [[ \
+            ${{ startsWith(env.TARGET_BRANCH, 'backports/') && 1 || 0 }} == 1 \
+          ]]; then
+            IMAGE_ID=$(echo "$TARGET_BRANCH" | sed -e 's,^backports/,,')
+            echo "image_id=$IMAGE_ID" >> $GITHUB_OUTPUT
+          else
+            echo "image_id=latest" >> $GITHUB_OUTPUT
+          fi
+
+  test:
+    needs: select
+    runs-on: ubuntu-latest
+    container:
+      image: ghcr.io/gsconnect/gsconnect-ci:${{ needs.select.outputs.image_id }}
+    steps:
+      - uses: actions/checkout@v4
+      - name: Prepare
+        run: |
+          meson --prefix=/usr \
+                --libdir=lib/ \
+                -Dgnome_shell_libdir=/usr/lib64/ \
+                _build
+
+      - name: Uninstalled Tests
+        run: |
+          meson test -C _build --suite gsconnect:data \
+                               --suite gsconnect:lint \
+                               --print-errorlogs
+
+      - name: Installed Tests
+        run: |
+          ninja -C _build install
+          xvfb-run -a dbus-run-session -- \
+          gnome-desktop-testing-runner gsconnect -L _build/meson-logs
+
+      - name: Upload Test Logs
+        if: ${{ always() }}
+        uses: actions/upload-artifact@v3
+        with:
+          name: test-logs
+          path: _build/meson-logs
+
+      - name: Test Build
+        run: |
+          ninja -C _build make-zip
+
+      - name: Stage packaged extension
+        run: |
+          ninja -C _build make-pkgdir
+
+      - name: Create and upload packaged artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: gsconnect@andyholmes.github.io
+          path: _build/gsconnect@andyholmes.github.io/
+