Fixes #4. remove_all_images now uses the images names in the DOCKER_IMAG...

...E map instead of hardcoded values.

I've added an "ORDER" entry for each image in the DOCKER_IMAGE map, so we can define the dependencies of the docker images and the order in which they need to be built (and removed). I've changed the remove_all_images function to now use the image names in the DOCKER_IMAGE map and the new ORDER attribute to determine which images need to be deleted and in which order (not doing the ordering gave problems when I wanted to delete an image on which another container indirectly depended. For example, deleting the "fsw" image is impossible when there is a container that still depends on "heise_fsw", because "heise_fsw" extends "fsw").

The function is a bit more complex than I wanted, because I couldn't find a proper way to iterate over a map in reverse order in bash ..... So, if anyone knows how to do this in a better way .... ;-)