To build the AppImage, you basically just have to:

    1) Build the base docker container, with something like
       # docker pull centos:6.8
       # docker build .
       # docker images (should show the image ID of the newly created image)
       # docker run <image id>
       Once created, you can just detach/attach and start/stop the container, i.e.
       # docker start <container id>
       # docker attach <container id>
       Building the container will take several hours at least.

       --- OR ---

       Get Sven's pre-built version:
       # docker pull scummos/centos6.8-qt5.7

    2) Copy the script into the docker container, using docker cp.

    3) Run the script.
       Running the script will also take quite a while the first time you do it,
       so it is advisable to always re-use the same container.

    4) Copy the resulting AppImage out of the container using docker cp.
