Docker images for building Flatpak

These Docker images contain the necessary bits for running flatpak-builder and generating a Flatpak application repository or bundle.

They are divided into the following layers:

• base/ - the base Docker image, containing the necessary bits for
  running flatpak-builder
• fdo-1.6/ - a Docker image containing the Freedesktop 1.6 Platform
  and SDK runtimes
• gnome-3.24/ - a Docker image containing the GNOME 3.24 Platform
  and SDK runtimes