contrib/docker/ubuntu.template

test: add a push race case where the raced push touch multiple heads

We check case where the raced push update all heads while the racing push
update one of them.

FROM ubuntu:__CODENAME__
RUN apt-get update && apt-get install -y \
  build-essential \
  debhelper \
  dh-python \
  devscripts \
  python \
  python-all-dev \
  python-docutils \
  zip \
  unzip