Add workflow to build arm64 flatpak builder image

.github/workflows/image.yaml

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+name: Flatpak images
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  flatpak:
+    name: Docker
+    runs-on: ARM64
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      - name: Login to Docker Hub
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          platforms: linux/arm64
+          push: true
+          tags: ghcr.io/${{ github.repository_owner }}//flatpak-github-actions:gnome-45-arm64

Dockerfile

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+# syntax = docker/dockerfile:experimental
+FROM fedora:latest
+
+RUN dnf update -y && \
+    dnf install -y dbus-daemon flatpak flatpak-builder git-lfs python3-aiohttp python3-tenacity python3-gobject xorg-x11-server-Xvfb ccache zstd libappstream-glib && \
+    dnf clean all
+
+RUN flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
+RUN flatpak install flathub org.flatpak.Builder -y
+RUN --security=insecure flatpak install -y --noninteractive flathub org.gnome.Platform//45 org.gnome.Sdk//45
+
+ADD --chmod=755 https://raw.githubusercontent.com/flatpak/flat-manager/b07fe8652357ce16d9625f72f9acceaca94d860a/flat-manager-client /usr/bin