message: Pin plumbum and six in Ubuntu 18.04 build dockerfile
diff: @@ -19,7 +19,7 @@ RUN apt-get update && \
  RUN pip3 install -U \
  pip \
  twine \
  - plumbum \
  + plumbum==1.6.7 six==1.12.0 \
  deb-pkg-tools
  RUN apt-get update && apt-get install -y --no-install-recommends \
  