This is caused by Docker bug 6345 which is already closed, but for some
reason the problem still persists. Closes GH-181. Patch contributed
by Michael Zedeler.
The ADD command will create /build automatically, so the RUN mkdir step
can be safely removed. Also, this has the benefit of reducing the number
of steps in the Dockerfile which is helpful for not as quickly hitting
the 127 layer limit.