WIP breaking stuff: updating docker image build process for Python 3.6

This requires a new way to pass requirements.txt files to Docker (since
they now link to each other), as well as building Python ourselves (since
even Ubuntu 16.10 doesn't have a decent Python 3.6).

This is just a WIP commit, will be fixed soon(ish).

docker/2_build/build-wheels.sh

@@ -1,3 +1,6 @@
 #!/usr/bin/env bash
 
-. /data/venv/bin/activate && pip wheel --wheel-dir=/data/wheelhouse -r /requirements.txt
+set -e
+cd /data/topdev/blender-cloud
+source /data/venv/bin/activate
+pip wheel --wheel-dir=/data/wheelhouse -r requirements.txt