Docker image improvement

[Ovidiu-Florin BOGDAN]

Hello,

I'm proposing some changes to the Dockerfile.
Considering the Docker guides, it is best to split the initialization into multiple RUN commands so that caching can be used.
I've done that and some other improvements and commited them to a branch of the androidbuilder repo, since we don't have this repo in a review platform.

Branch is here: https://quickgit.kde.org/?p=scratch%2Fcordlandwehr%2Fandroidbuilder.git&a=shortlog&h=a259aa7fef4849f70970a8e9fa82f8c467747e9d[1]

Please provide feedback.-- 
*Ovidiu-Florin Bogdan*
GeekAliens.com[2] 
Kubuntu Romania Representative[3]
_Kubuntu Council Member_

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[1] https://quickgit.kde.org/?p=scratch%2Fcordlandwehr%2Fandroidbuilder.git&a=shortlog&h=a259aa7fef4849f70970a8e9fa82f8c467747e9d
[2] http://ovidiu.geekaliens.com
[3] http://kubuntu.org
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