contrib/docker/debian-jessie

doc: prevent non-literal text block from being treated as literal one

This creates hg.1.html as expected.

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