Dockerfile: added python3.6

[pwalkr]

This is needed to compile latest python for our devices.

I tried replacing all of the python things with python3, but python 2 ended up being installed anyways.

[DavidZemon]

Scons is a Python-based build system and is the likely culprit of your Python 2 dependency. What if we just drop scons out of the image? Any reason it's there in the first place?

I also had issues with m2crypto - couldn't install from apt and pip3 fails to build it.

So, if scons and m2crypto are unnecessary, great! Remove them and jump to 3. Otherwise, sounds like we're installing both for a while.

[pwalkr]

That sounds like another whole day of building docker images... I was actually in the middle of a test to see if python3.6 was enough without any python or python3 symlinks, when my laptop froze. I can give it one more try tomorrow on my desktop. Won't merge until I get a full s5t image built.

[pwalkr]

npm and other things also depend on python2.7. It can stick around for now.

[DavidZemon]

Can you rebase and then bump the docker-native script to reference v0.3.0?