A flatpak package is available in Debian Buster and newer. To install it, run the following as root:
apt install flatpak
For Debian Jessie and Stretch, a flatpak package is available in the official backports repository.
If you are running GNOME, it is also a good idea to install the Flatpak plugin for GNOME Software. To do this, run:
apt install gnome-software-plugin-flatpak
Flathub is the best place to get Flatpak apps. To enable it, run:
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
To complete setup, restart your system. Now all you have to do is install some apps!
Note: graphical installation of Flatpak apps may not be possible with Alpine.