Mattermost Desktop - Flatpak

Prerequisites

$ flatpak install flathub org.freedesktop.Sdk//18.08
$ flatpak install flathub io.atom.electron.BaseApp//18.08

Build to local repository, install

$ flatpak-builder build-dir com.mattermost.Desktop.json --force-clean --repo=repo
$ flatpak --user remote-add --no-gpg-verify test-repo repo
$ flatpak --user install test-repo com.mattermost.Desktop