Install the runtimes
flatpak remote-add --from gnome https://sdk.gnome.org/gnome.flatpakrepo
flatpak install gnome org.freedesktop.Platform//1.6 org.freedesktop.Sdk//1.6
Local building
flatpak-builder NixView org.gnode.NixView.json
flatpak build-export repo NixView
Add local remote
flatpak remote-add --user --no-gpg-verify nixview-repo repo
flatpak install --user nixview-repo org.gnode.NixView
Run the Flatpak
flatpak run org.gnode.NixView