Modified Dockerfile to reduce resulting imagesize (currently about 250 MB)

Hi,

I modified the Dockerfile a little bit for building Docker images for Shinobi to reduce the resulting imagesize. Right now the modifications result in an image that is about 250MB less in size compared to images without these modifications. Let's have a look at docker image ls output.

$ docker image ls
REPOSITORY                                   TAG                 IMAGE ID            CREATED             SIZE
contribreduceimagesize_shinobi               latest              05163c80408c        7 minutes ago       854MB
registry.gitlab.com/migoller/shinobidocker   add-gitlab-ci       44857acd967e        2 hours ago         1.06GB
migoller/shinobidocker                       latest              865533f4ac6d        14 hours ago        1.06GB

The modifications to the Dockerfile just reduce the amount of layers of the images by grouping commands, eg. installation of ffmpeg, to fewer intermediate containers and layers.

I did not modify the Dockerfile's content, package installations, etc. at all !

That's it. Feedback appreciated.

Regards, Michael