This replaces bash with the docker command, freeing memory and
automatically returning the exit code of the docker command as the exit
code of the shell script.
1_base: builds a base image, based on Ubuntu 16.10
2_buildpy: builds two images:
2a: an image that can build Python 3.6
2b: an image that contains the built Python 3.6 in /opt/python
3_buildwheels: builds an image to build wheel files, puts them in ../4_run
4_run: the production runtime image, which can't build anything and just runs.
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).
