Decrease the amount of Dockerfile commands

[nanory]

What would you like to be added:
After reviewing the Dockerfiles in this repo, I noticed that it uses a lot of Dockerfile commands for similar tasks (e.g. installing applications etc) which lead to new container layers.

By combining comannds, you can decrease the amount of layers it creates.

Why is this needed:
Some design patterns suggest to keep the container layers as low as possible. We could use this approach to also update the Dockerfiles and add some more comments to it, so it's clear for externals what this containers contains and why it needs specific packages for example.

[nanory]

First adjustments have been done in #4 . This change is currently in review