Flatpak: Add CI workflow

.github/workflows/build_flatpak.yml

@@ -0,0 +1,43 @@
+name: Build latest (Flatpak)
+on: [push]
+
+concurrency:
+  group: ${{ github.ref }}-flatpak
+  cancel-in-progress: true
+
+jobs:
+  build:
+    if: github.ref_name == github.event.repository.default_branch
+    runs-on: ubuntu-latest
+    container:
+      image: bilelmoussaoui/flatpak-github-actions:freedesktop-23.08
+      options: --privileged
+    strategy:
+      matrix:
+        arch: [x86_64, aarch64]
+      # Don't fail the whole workflow if one architecture fails
+      fail-fast: false
+    steps:
+    - uses: actions/checkout@v4
+    # Docker is required by the docker/setup-qemu-action which enables emulation
+    - name: Install deps
+      if: ${{ matrix.arch != 'x86_64' }}
+      run: |
+        dnf -y install docker
+    - name: Set up QEMU
+      if: ${{ matrix.arch != 'x86_64' }}
+      id: qemu
+      uses: docker/setup-qemu-action@v2
+      with:
+        platforms: arm64
+    - uses: flatpak/flatpak-github-actions/flatpak-builder@v6
+      with:
+        bundle: net.classicube.flatpak.client.flatpak
+        manifest-path: misc/linux/flatpak/net.classicube.flatpak.client.yml
+        arch: ${{ matrix.arch }}
+
+    - uses: ./.github/actions/notify_failure
+      if: ${{ always() && steps.compile.outcome == 'failure' }}
+      with:
+        NOTIFY_MESSAGE: 'Failed to compile Flatpak build(s)'
+        WEBHOOK_URL: '${{ secrets.WEBHOOK_URL }}'