The supplied flatpak manifest org.cataclysmdda.CataclysmDDA.json will download and build the latest release of Cataclysm-DDA. An app built in this way should be available on flathub:
https://flathub.org/apps/detail/org.cataclysmdda.CataclysmDDA
Check the flatpak documentation at http://docs.flatpak.org for information on how to customise the manifest, build your own copies of the app, and install or distribute them.