Add current flatpak to Ubuntu image

We need flatpak in the Ubuntu image(actually any linux distribution would do it but so far I see Ubuntu 16.04. has already packages)

sudo add-apt-repository ppa:alexlarsson/flatpak
sudo apt update
sudo apt install flatpak flatpak-builder

It is the official repository of flatpak: https://flatpak.org/getting.html

It would nice too that the actual runtime is already installed so we don't need to download it all the time:

flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak install flathub org.kde.Sdk//5.9 # currently used
flatpak install flathub org.kde.Sdk//5.10 # used in the near future