NZBGet could use Python and unzip support

[mbirth]

I'm currently evaluating NZBGet on my Synology. Now that I'm playing around with extension scripts, I found that many of them are written in Python (2.x) which isn't installed in this Dockerfile. Also, I can't even write a simple script to unzip zipped NZBs because there's no unzip or a full 7za installation (7zr/p7zip doesn't support zip files.).

[mbirth]

Thinking about it, couldn't you just add an environment variable to the apt-get install line so that we can set our own packages via the -e parameter?