Make docker image smaller by using Precise

Changing to precise means we don't have both python 3 and 2 installed in
the image, only python 2. Also do some cleanup at the end of the build
since we are exporting the final filesystem, and don't care about the
size of the intermediate layers.

Change-Id: Iac61d4a882aee19214de5dd74bbbaacc8b0f3414
Merge "Make docker image smaller by using Precise"