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.
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).
