To test a PR, I downloaded the docker sdk and tried following the example listed there, but it does not work, even before making any changes.
The buildsystem docs mention a slightly different sequence, including "install -a".
Anyhow, the failure is a simple mkdir error under /builder/bin which is owned by root, not buildbot (1000). The docs however mention explicitly that everything should be done as unprivileged user.
What gives?