I figured out where the misunderstanding is:
$ docker run --rm --tty --interactive netlify/build:xenial /bin/bash
buildbot@9db34dc599d9:/$ ls /opt/buildhome/
python2.7 python2.7.11 python3.5 python3.5.6 python3.7 python3.7.2
buildbot@9db34dc599d9:/$ exit
$ docker run --rm --tty --interactive netlify/build /bin/bash
buildbot@a6087d395f0f:/$ ls /opt/buildhome/
python2.7 python2.7.5 python3.4 python3.4.0 python3.5 python3.5.5 python3.6 python3.6.4
I thin the instructions in “Want to try to make your own sausage?” here are incorrect: https://www.netlify.com/blog/2016/10/18/how-our-build-bots-build-sites/.
The reason is that despite pulling netlify/build:xenial, when I run your ./test-tools/start-image.sh, it runs netlify/build which is not the same thing.
My guess is that ./test-tools/start-image.sh is out of date in that it just presumes the legacy build image. It’s worth asking how come nobody noticed this; are engineers not actually using start-image.sh, or not using xenial, or not using the combination?