Swap docker container for flatpak run

Author: Motobug

.github/workflows/ci.yml

@@ -195,7 +195,7 @@ jobs:
     name: Flatpak Build
     runs-on: ubuntu-latest
     container:
-      image: bilelmoussaoui/flatpak-github-actions:gnome-nightly
+      image: ghcr.io/flathub-infra/flatpak-github-actions:gnome-48
       options: --privileged
     steps:
     - uses: actions/checkout@v2
@@ -209,7 +209,6 @@ jobs:
         rm -rf /usr/share/dotnet
         rm -rf /opt/ghc
         rm -rf /usr/local/share/boost
-        docker system prune -a -f
         echo "Disk space after cleanup:"
         df -h
     - name: Setup Packages