You'll just have to use this image as base for your own-backed Dockerfile.
This image was designed for continuous integration on TravisCI, where the only way to run flatpak is inside a docker container.
It doesn't came with provisioned Runtimes nor Sdk's to limit the images size, so you still have to install the ones you really need.
For exemple, if you plan to target only Gnome 3.16 & 3.18 runtimes, just run :
docker pull lcallarec/flatpak-gnome docker run lcallarec/flatpak-gnome flatpak install gnome org.gnome.Platform//3.16 org.gnome.Platform//3.18