I like to run my Linux as an operating system, so I usually resort to toolbox for packages and development. However flatpak-builder is distributed as a flatpak itself, so here's how you can go about building flatpaks yourself for when GNOME Nighlies are not enough.

On GNOME OS, developer tools like git and toolbox aren't part of the base image. You get them by enabling the developer system extension:

sudo updatectl enable devel --now

This gives you the toolchain needed to clone repos and build things locally.

Installing flatpak-builder

The builder is distributed on Flathub as org.flatpak.Builder. Install it like any other Flatpak:

flatpak install flathub org.flatpak.Builder

Building and Installing Locally

Here's how I build Shaper, an icon designer for GNOME symbolics.

flatpak run --command=flatpak-builder \
org.flatpak.Builder --user --install \
--force-clean build-dir org.gnome.design.Shaper.json

And that's it!