Install dependencies:

Fedora:
sudo dnf install flatpak-builder

Ubuntu:
sudo apt install flatpak-builder

Build + install (for testing):

flatpak remote-add --user --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
flatpak-builder --force-clean --user --install-deps-from=flathub --repo=umu-repo --install umu-launcher org.openwinecomponents.umu.umu-launcher.yml

Remove

flatpak --user remove umu-launcher

Usage examples:

winecfg:

flatpak run --env=GAMEID=umu-starcitizen --env=WINEPREFIX=/home/tcrider/Games/umu/umu-starcitizen org.openwinecomponents.umu.umu-launcher winecfg

running a game using the default latest UMU-Proton:

flatpak run --env=GAMEID=umu-starcitizen --env=WINEPREFIX=/home/tcrider/Games/umu/umu-starcitizen org.openwinecomponents.umu.umu-launcher /path/to/some/game.exe

running a game using the latest GE-Proton:

flatpak run --env=GAMEID=umu-starcitizen --env=WINEPREFIX=/home/tcrider/Games/umu/umu-starcitizen --env=PROTONPATH=GE-Proton org.openwinecomponents.umu.umu-launcher /path/to/some/game.exe

running a game using a specific proton version:

flatpak run --env=GAMEID=umu-starcitizen --env=WINEPREFIX=/home/tcrider/Games/umu/umu-starcitizen --env=PROTONPATH=GE-Proton9-1 org.openwinecomponents.umu.umu-launcher /path/to/some/game.exe