Reasons for making this change

Flatpak is a widely-used Linux application packaging format that's available for install on basically every Linux distribution, and is installed out-of-the-box on many, including all Linux distributions in the Red Hat family and many distributions downstream of Ubuntu.

Links to documentation supporting these rule changes

Cribbed directly from the Element Flatpak's .gitignore file at https://github.com/flathub/im.riot.Riot/blob/master/.gitignore, simply because Element is a pretty popular application. (Important bit of context for those not familiar with Flatpak/Flathub packaging: the workflow for getting an app into Flathub, the de-facto public Flatpak repository, puts packaging files in a separate repository from the actual app.)

Note in particular that this covers all directories that will be created by the Flatpak tutorial's suggested flatpak-builder command: https://docs.flatpak.org/en/latest/first-build.html#build-and-install

repo is the OSTree repository that the built Flatpak will be "published" to for testing, .flatpak-builder is an internal build directory, and builddir is another, user-specified build directory.

If this is a new template

Link to application or project’s homepage: https://flatpak.org/

Merge and Approval Steps

• Confirm that you've read the contribution guidelines and ensured your PR aligns
• Ensure CI is passing
• Get a review and Approval from one of the maintainers